Earlier this year at CES, HP introduced its HP Stream Mini ($180) and Pavilion Mini ($320 and $450) mini-desktops. They’re small, inexpensive, and in the case of the Stream, silent. They turn out to be surprisingly upgradeable as well. Ars Technica has details and benchmarks, but of course I have my own priorities based on their discoveries.
My first priority would definitely be to upgrade the memory to at least 8 GB, but if you’re doing that, you might as well spend $50 more and go all the way to 16 GB. By the time you really need 16 GB, DDR3 memory may be more expensive than it is today. A good-quality Crucial 8 GB SODIMM runs about $60 right now, so a pair of them would run $120. If you can’t afford that in a single purchase, buy a single 8 GB SODIMM, take the performance hit, and replace the factory-installed SODIMM with a second matched 8 GB SODIMM when you can afford it.
My other order of business would be the SSD. All of these machines include an M.2 port, but since it’s SATA M.2 rather than PCIe M.2, there’s no real speed advantage. You could potentially get better performance by putting a higher capacity drive in the SATA bay, since the workload is spread out over more chips. But it’s nice to have two options for adding SSDs.
Ars also has a method for upgrading the wi-fi to put it on 5 GHz 802.11ac. I’d rather put that $40 toward more memory or SSD storage, but if fast wi-fi is a priority for you, it’s nice to have the option.
The other thing I would do is install Windows 10 the day it’s released. A clean install, of course, to get rid of the bloatware.
So, Celeron, Pentium, or Core? For the price, I’d probably go with the $180 Celeron model. The other machines are faster, but a Celeron with lots of memory and a fast SSD is still a very capable machine, and you could deck out the Celeron and be out less than the cost of the Core-based model.