2GB RAM should be enough for now - you'd be very hard pushed to use that amount.
You could also consider WD Raptor hard disks - these are very fast but more expensive and have less capacity than 7200 rpm disks. Or the new Seagate Barracuda 7200.10 drives which use perpendicular recording and are faster than traditional 7200 disks.
I would go with AM2 or wait for Conroe but your obivously not doing that. As for the power supply get atleast 500w because of the crossfire setup i dont know exactly how much but atleast 500w and dont go cheap on them either. =]