I'd definitely get a SATA HDD. That'll speed things up nicely! May as well since you're spending $1500 on a PC. That's certainly not a tight budget if ya ask me.
I'd get the Radeon X800 Pro if ya don't feel like waiting. The XT hasn't been benchmarked yet...only the XT Platinum Edition. The regular XT will be a step below the Platinum Edition, so you may as well just get a Radeon X800 Pro. They're "only" $400 USD.
That RAM will do fine, but I'd spend more and get 768 MB or 1 GB. 512 MB simply isn't enough with today's games. Why can you only afford 512 MB if you have $1,500 to spend?
The mobo and processor look great. If you combo together Corsair XMS RAM with a Radeon X800 and Athlon 64 then you'll have an incredibly fast PC that'll run games for years to come. Good luck!
-Lasereth