Help with upgrade

Since it won't be till after I'm done with school (2 years) that I can get a pc from this site I'd like to upgrade my existing pc so I need some advice. I have about $300-$350 to spend. I mostly play MMO's while listening to music. So looking for something that can play the new stuff coming out (Aion, Champions Online, Star Trek) Doesn't have to run on High Settings, medium is fine with good frame rates. Here's what I need:

Motherboard(Micro or ATX)
CPU
RAM
Video Card


Couple of Notes:

Case is a pre-built Mid-Tower. It only has 1 120mm fan for exhaust. I can mod case for 1-2 80 mm fans for extra cooling if needed.Plan to overclock cpu so if I need an after market heatsink and fan let me know. My current pc has a AMD socket 939 motherboard so if I can use it I will. There's a couple of AMD dual-cores that will fit but the ram is DDR PC3200 so I think that's where its going to bottleneck. 17" monitor (1280x1024) so no SLI or Crossfire needed. Using Xp so should only need 2gb of ram I know XP will recognize 3gb So if it will make noticeable performance increase Ill buy it. Well that's it if you need any other info let me know. Thanks guys.