I upgraded a Compaq Evo D51S today. This was also sold under the name D510, and may have also been sold under the HP or Hewlett Packard brand. It was intended to be a low-profile, relatively affordable business computer.

Upgrading it poses some challenges, but there are some things you can do with it.

This one has a 2.0 GHz Celeron in it. It will support a 2.4 GHz P4 without any issues (and a lot of them were sold with that chip), but I think that's as high as you can go with the CPU.

The 2.0 GHz Celeron that came in this system will bog down with a heavy Photoshop filter and I'm sure some of the things I do in Adobe Premiere would bring it to its knees at times, but if your primary use of the machine is word processing, spreadsheets, web browsing and e-mail, it's plenty fast. I would max out the system RAM before I replaced the CPU.

You can forget about motherboard replacements in this machine. Everything about the motherboard inside is odd, to get everything to fit in a smaller case. Compaq used to be criticized (sometimes unfairly) for using proprietary motherboards, but this one's definitely proprietary.

Inside, you're limited to two DIMM slots. I pulled the memory and replaced it with a pair of PC2100 DDR 1 GB DIMMs, which is the maximum the system supports. According to Crucial, PC3200 memory is compatible. Of course if you're buying new memory, it makes sense to buy the faster stuff, in case you ever want to put the memory in another system.

As I write, 2 GB of RAM sells for about $120. That's more than these computers sell for on the used market but more memory is probably the best thing you can buy for one of these machines, especially if it came with 256 MB of RAM. Whether you really need 2 GB depends on what you intend to do with the computer. For a lot of people, 512 MB would be adequate (Windows XP doesn't run all that well in 256 MB of RAM anymore). But if memory prices fall, maxxing out the memory in this machine is a no-brainer. Even at current prices, I still think it's justifiable.

The onboard video is the Intel 845G integrated video. It was better than I expected, but it steals system memory and, at least theoretically, it reduces memory bandwidth. The AGP slot is oriented vertically, so there's only room for a low-profile card. That limits your choices somewhat. I had a low-profile ATI card with a Rage 128 Pro chipset on it. It's not the most exciting card in the world, but it's better than the integrated Intel video and it freed up some system memory for me. For what I want to do with this system, it will be fine. I'm not sure that Sid Meier's Railroads! will run on it, but Railroad Tycoon 3 will, and from what I understand that's the better game anyway.

There are a number of low-profile AGP video cards on the market that would be a suitable upgrade for this machine. None of them are cutting edge, but there are a few that are DirectX 9-capable, and prices range from $20 to $40. The built-in video is adequate, and while my first impression of it was that it didn't bog the system down nearly as badly as the integrated video in the P3 days did, I'm still not a big fan of it. I think adding a discrete video card is a good move.

The stock Seagate Barracuda 7200.7 is a pretty good performer. At 40 GB it's relatively small, and it won't keep up with a brand-new drive, but for a lot of uses it's plenty fast. From what I understand it will support hard drives larger than 137 GB but you may have to mess with IDE modes in the BIOS to make it happen. The trick appears to be to set the BIOS to use bit shift instead of LBA. Additionally, you have to be running Windows 2000 SP4 or XP SP2 to see the full capacity of the drive. I don't have a large drive to put in it, so I haven't tested that.

There's no room for a second drive in there, so if you want additional storage beyond what's already there, it will have to be external. That's the price you pay for a low-profile system.

The CD-ROM drive in my particular unit was pretty balky. I'm going to replace it with a CD-R/RW drive for the short term, and eventually (probably early next year) put a DVD burner in it. I'm primarily interested in putting home movies on DVD. For backup and data transfer, I pretty much use USB flash drives exclusively now. They're a lot faster and more convenient than messing around with CD/DVD burning software.

Speaking of USB, the USB ports all seem to be USB 2.0, which is nice (installing software off a USB 2.0-based flash drive makes you want to swear off optical media forever), but the ports on the front are recessed far enough that only a standard cable or a very low-profile flash drive can plug into them. My SD reader would only plug into the back, which is inconvenient.

The system has two full-size PCI slots for expansion. I put an IEEE 1394 (Firewire) card in one of the slots, since I want to do some light video work with it. The other slot will probably get an 802.11b wireless card. If I needed that PCI slot for something else, I could plug in a USB adapter for wireless networking.

I used to be in the habit of buying the biggest case I could afford or find (they weren't always the same thing), so a really low-profile desktop like this Evo 510 feels a little strange. But a lot of things are different now. I could put a 1 TB hard drive in this system if I needed an obscene amount of storage. USB ports eliminate the need for Zip or Jaz or Syquest drives and even, to a large extent, for CD or DVD burners. If it weren't for my interest in video, I wouldn't bother with a burner in this machine at all. And since sound and networking are built in, there's no need for a lot of expansion slots. It would be nice to have three PCI slots instead of just two, but I would imagine a lot of people never even fill two.

As it is, this computer fits on a small desk, and if you put an LCD monitor on top of it, the combination will take less real estate than a 17-inch CRT monitor does.

There are a lot of these machines on the market now, either coming off lease or being replaced due to business upgrade policy. They're cheap ($75-$150 depending on configuration) and I think they make an excellent home PC. They're cheap, unobtrusive, and surprisingly expandable.

A decked-out 510 probably won't run Vista all that well, but a lot of new PCs don't run it very well either. I think a 510 running Windows XP or Linux can be a very useful computer for a good number of years.