I’m looking at building myself a new PC for the first time in years. That’s a little bit of a misnomer though. Today, building a PC can mean bolting as few as two components into a case and connecting four cables. Building PCs in the 1990s was a lot more difficult. I remember in 1994, during one of my first builds, someone walking past in the hall, looking at the mess of cards and cables, and asking, “How do you know which one goes where?”

Today, the assembly is pretty easy. Figuring out what to buy is harder. In 1994, the differences between the various flavors of 386 and 486 chips available was confusing, but it all fit on an index card. Mainly the difference came down to the amount of memory the chip could address (386) and whether it had a math coprocessor (486). Beyond that all you really had to worry about was clock speed. Back then the research took 30 minutes and the system took hours to build.

Today there are two chip manufacturers (down from four) but they both have half a dozen product lines. And nobody really talks about clock speed anymore. That’s fine because clock speed was a crude measure of performance, but is throwing numbers like 560 or 840 or 965 on the chips really any better? Today the research takes hours (if not days) and the system goes together in about 5 minutes. Shake the bag right and it could just come out of the bag fully assembled.

I’m really only looking at two chips: the AMD Phenom II x2 560 and the AMD Phenom II x4 840. I can get either chip bundled with a decent motherboard for under $99. Both are intended as lower mid-range CPUs. Better than a Celeron, but not intended to satisfy people who want the highest gaming benchmarks or people who are trying to run high-end scientific or engineering applications.

When you choose between the two, you’re trading cores for cache. The 560 has 6 MB of L3 cache and two CPU cores, while the 840 has four cores and no cache. The 560 also runs 100 MHz faster, but in the 3 GHz neighborhood, a difference of 100 MHz is going to be tough to notice.

Last summer I spent a couple of days with an AMD 640, which despite the name is basically the same CPU as the 840 but clocked 200 MHz slower, at 3 GHz. Using it didn’t make me feel deprived. Not in the least.

Reading the user reviews at the usual places, it’s hard to find anyone with a legitimate complaint about either the 560 or 840. You find a small number of DOAs and of course those people will be upset, but that’s a chance you take with any chip. You find some complaints with the bundled heatsink/fan, but you expect that too. Beyond that, the complaints you find are about the name being confusing or not being able to unlock cores or overclock.

I dismiss the latter case. If you wanted a $150 CPU, don’t buy a $100 CPU and gripe when that’s all you get. Sometimes the cores are disabled because they weren’t usable. It’s better than throwing the chips away.

I asked around a bit, and Gatermann pointed me to a benchmark  that tried to isolate the difference L3 makes. It found the difference was anywhere from 5-20%. And for the kind of work that I would want the higher performance, like audio and video encoding, there’s not enough difference to be noticeable. Theoretically, at clock rates over 3 GHz the difference is larger. But still, not a lot.

Unfortunately I wasn’t able to find a similar benchmark that showed two otherwise identical AMD CPUs with differing numbers of cores for comparison.You’d think someone would have done it–all they have to do is benchmark a Phenom II x2 500-series chip and its equivalent Phenom II x2 900-series chip. Based on the benchmarks I could find, sometimes the difference between a 2-core and a 4-core was nearly 100 percent and sometimes it was more on the order of 25-30 percent, but frequently it could also be less than 10 percent. Part of the difficulty is finding software that takes advantage of the extra cores, and there’s still a lot of software out there that doesn’t. The more heavily you multitask, the more benefit you’re going to see.

I suppose if there was a big difference in performance and it was easy to demonstrate it, there would be a comparable difference in price. There’s not a big difference, so that’s why a 2-core CPU with L3 cache sells for $90 and a 4-core CPU without L3 cache sells for $100.

This is pure speculation on my part, but I would think in the future, there will be more software taking advantage of the extra cores. So the system with more cores might be a little bit more future-proof. But I don’t think I’m going to fret over the difference. I’m planning to make a purchase this weekend. Thanks to back-to-school promotions, prices will be a bit lower, but selection will be a bit more limited. I’ll buy what I can get, because either one will be a big improvement over what I have now.

Judging from the always-limited selection of the Phenom II x4 965, which has both L3 cache and 4 cores for about $40 more, it seems a lot of people are willing to pay a little extra to get both and hedge their bets. You’re paying roughly a 40% premium for perhaps 30% greater performance–it has a higher clock rate too–which goes against my general tendencies, but I can understand the logic. If you don’t intend to upgrade CPUs later and can stretch another 3-6 months out of the system, you might get $40 in additional value out of it.

But for the time being, I’m content to hang out in the low end.