I recently added an ATI HD3870-X2, an extra hard drive, more RAM, and also installed Vista-64.
Since then I've had to reduce the amount that I overclock my CPU from 8x375MHz to 8x353 MHz. Could the power supply be the limiting factor? It shouldn't be the RAM as it's rated at DDR-800. Temperature is fine. The issue I get is freezing in games, random restarting, and blue screen. Seems to happen more frequently when playing games.
Current hardware:
Core2Duo E6400 @ 2.9 GHz
Abit AB9
4 GB RAM
ATI HD3870-X2
3 7200rpm SATA HD
1 IDE HD
1 SATA DVD-RW
X-Mystique Sound card
3 x 120mm fans + 80mm CPU fan
Vista Ultimate 64bit
Hyper Type-R 550W PSU