Don't get a 300 watt power supply unless you're running a first generation Athlon or something. With those specs you got now, I suggest a 450watt power supply. The CPU is badass and so is the video card. The RAM needs work because it won't work with your Athlon system. You need to use DDR not DDR2 memory. Get a OCZ or Corsair DDR400 RAM. THat case is so hot, I'm literally drooling a bit right now. Everything looks good though. Change the power supply and the RAM:
CORSAIR XMS 1GB (2 x 512MB) 184-Pin DDR SDRAM Unbuffered DDR 400 (PC 3200) Dual Channel Kit System Memory - Retail
ATX 2.01 Upgraded, Dual 12V Rail Power Supply
Rosewill RV450-2 ATX 2.01 450W Power Supply 115/230 V UL, CSA, TUV, FCC - Retail
I didn't know what your budget is so I picked what was affordable. But they do look good though.