I thought I might come in and say a few things, considering I visit TomsHardware almost daily. There are

of discrepancies in this thread, and I hope to clear many of them up.
First of all, as of now, there is no reason to get PCIe over even 4x AGP for a mid-level PC. It's not practical at all. Also, computers have been in a transitional phase for a while, but nothing has been solidified yet. It would be a big mistake to buy current technology at this point if you want something that will last (SKT754 proves that).
Anyway, here are some details. The former Socket A has "unofficially" been re-named Socket 462, and Socket 939 has "unofficially" taken the name of Socket A. Socket 462 is being phased out, but there are still plenty of suppliers and backstock. This tech absolutely hasn't been abandoned yet.
AMD spent many a dollar on SKT754, it iwas meant to be the interface for mainstream 64bit CPUs. However, they knew it would be a mistake to seperate mid-level and upper-level CPUs by socket. SKT940 was used initially for the Opteron series of server CPUs. Later, as the AMD64 CPUs were introduced, the upper-end FX CPUs were aimed at using SKT940. However, the segregation problem was taken into consideration, and now SKT940 is solely used for Opteron CPUs. Enter SKT939, the new "Socket A". This interface was designed to take the common of both A64 and AFX CPUs and provide a common ground for people who wish to use 64bit processing.
The
Athlon64 is just a regular 64bit CPU. It has the standard 64-bit extensions, a good amount of cache, and a simple architechture. Currently uses SKT754 and SKT939, although SKT754 A64s are no longer in production.
The
AthlonFX is AMD's flagship 64bit CPU. This thing usually has a large amount of cache (1MB or 2MB), several failsafe features (it requires REG ECC memory, for example), and is based off the same architecture as the Opteron. Uses SKT939
The
Sempron is just an A64s that was stripped of the 64bit extensions and cache. To compare it with a duron would be appropriate. They are currently the sole user of SKT754
DDR has evolved over time, ofcourse, and now DDR-2 is the newcomer. However, just as DDR was in it's infancy, DDR-2 is not worth the extra money. Cheap DDR-2 has absurd response times. 4-6-6-10 response time isn't uncommon. Right now, there is no justification for using DDR-2 over DDR. It's too early in the game yet.
SATA is a good rock solid interface. I would not reccommend against it for any reason, except for compatibility issues with older OSes.
Anyhow, with that said, you could get a very nice system for ~$500 US. You dont need a GeForce 6 series card, so a GF4Ti 4200 would suit nicely ($80). The motherboard is the heart of the system, and you dont want to spare any expense for it. Get yourself a MSI K7N2 Delta or an ASUS A7N8X ($90-110 depending on the features (DONT GET INTEGRATED GFX, OR THE LITE SERIES!!)). Use MatthewPapa's idea and get a 3200+ ($140, buy retail). Also, get a good quality stick of 512 PC3200 ($60). You still have $100 budget left on a case. Buy whatever you want, it doesnt matter much. Just make sure it's ATX and has a PSU of atleast 400w.
I hope you find my long, rambling post informative. But knowing me, I probably got too far into the details. Sorry

.