After upgrading from a XFX 512MB 8600GT (300W min. req.) to a Zotac 512MB 9600GT ECO (350W min. req.) my system started to unexpectedly reboot.
Mobo: Intel DP35DP
CPU: Core 2 Duo E6750
OS: WinXP 64 Pro
PSU: 500W retail
I suspected it could be the PSU, but it's an "ECO" version of the 9600GT, and it supposedly require not too much power (350W) from the PSU.