ok...
A64 was originaly introduced on socket 754 for mainstream users and has now moved to socket 939 for dual-channel operation
Basically socket 939 should carry the platform until K9 (the next amd's cpu) is around.
Since I am not rich, and I cant afford to throw money away I am going to recommend a reasonable upgrade. I think it is fair to say that the next time you upgrade you will probably replace the motherboard due to SATA 300 and PCI-Express which will become mainstream within a year or 2. Of course I cannot tell you how often you upgrade, so I am just assuming here.
BTW, Socket 939 starts with A64 3500+ and right now goes to A64 3800+ (it also supports FX 939 processors)
Socket 754 goes from A64 2800+ to A64 3700+
If you do not upgrade frequently and will buy a new motherboard with your next cpu, go with socket 754 and do not waste 2-3 times the amount on socket 939.
A64 3200+ Newcastle 2.2ghz 512kb - $271
A64 performs better with higher clock rate than with more cache (this is esp true for gaming) and since you are not overclocking, might as well get the higher speed version of the 3200+
Chaintech VNF3-250 $88 - Silver choice award winner for socket 754 roundup on Anandtech - best overclocker (not that this matters to you)
get 1 gig of RAM - depending on your budget you could balance things and get Kingston Value ram or get something like
OCZ PC3200 Enhanced Latency 2-3-2-5 512mb $125 x 2 sticks
But you could always get similarly priced Corsair XMS series or other RAM
Case...well this depends on your preference and so on. You could go with cheap case and a good power supply like Antec 430, Thermaltake Butterfly 480 or even OCZ 420 (with new 24pin adapter and BTX compatible - so you could reuse it in the future)
Or for a quiet all-in one solution, get
Antec Sonata with 380 Watt PSU - $99
WD 74 gig Raptors are fast HD but they cost a lot and don't hold a lot of space. You can get one for 181 - $20 mail in rebate from Newegg. It is the fastest desktop drive. Otherwise, I'd probably get something like
Hitachi 160gb 8mb cache 8.5ms seek times 7200 rpm - $99 or Seagate if you prefer a quieter HD.
6800GT on special for $340
Since you are not looking to overclock and might look for longeviety somewhat (PS3.0 support) and since 6800 series beat x800xt in Doom 3, this is the clear choice for price/performance
Other components like DVD-writer, etc. are up to you, if you have any questions, just ask
I am not sure what your price range is....but you could always adjust things here and there.
EDIT: Just found the Chaintech motherboard on special in Hot Deal for
$69
Also you could always get a HD with a Mail in rebate from Fry's, COMPUSA, BestBuy, etc.
So you could save additional loot