First things first...
GAMES = AMD
SocketA is nearly dead, so forget about Athlon XP or Sempron.
Socket754 is not bad, but it's not the smartest choice, unless you want to keep your HIGH-ENG AGP card.
So, you should get a Socket939 based with PCI-EXPRESS for future upgrade.
SLI is mostly a budget oriented decision. If you really thinks that your budget will permit you 2 SLI-READY gpu (now or in a near future), go for it and pay the extra. But, the actual performance benefits is not very good, when you check price/performance ratio. If you buy 2 card at 200$, you will not get more FPS than getting 1 card 400$. But, if you buy 1 cars at 200$ today, and get the other for 100$ in a year, you will boost your performance for a bargain, but don't forget to offset that price with the added cost of a SLI enabled mobo, you must consider this in your decision too!
Since all new mobo supports SATA and SATA HDD are affordable, go for SATA if you need to buy a new HDD.
The minimum setup would be something like that :
- Athlon 64 3000+ S939 (you can overclock it easily)
- nForce4 motherboard
- GeForce 6600GT 128Megs PCI-EXPRESS
- DDR400 2 x 512 Megs
A kick-ass rig would look like this :
- Athlon 64 3500+
- nForce4 SLI motherboard
- GeForce 6800GT PCI-EXPRESS (SLI READY)
- DDR400 2 x 512 Megs
-
A7N8X / <font color=green><b>Athlon XP 1800+</font color=green> o/c to <font color=green>Sempron 2800+</b></font color=green>
Kingston DDR333 2x256Megs
<font color=red>Radeon 8500 128Megs</font color=red> @ C:275/M:290 <- <i>It's enough for WoW!</i>