beanstalk- Just plain resets or BSODs?
How long have you been using this rig?
Sounds like a typical psu issue.
Or maybe a h/w issue such as a bad OC (but that should BSOD ideally) or hardware incompatibility.
or an ATI vpu recover
-as you don't seem to report any BSODs
Some *ahem* games, bad downloads i.e. may cause random shutdowns.
Please monitor and record all system temps and voltages while running some cpu stress test like Orthos or IBT or something.
Monitor and record the same while running fur rendering staibility test.
go to BIOS.
Check cpu target temperature or cpu shutdown temperature and other system temps.
Either some system temp sets off auto shutdown- as we have in cpu overheat protection or the psu is acting funny or the gpu has an issue.
Last thing, try using only a single 2GB stick and test the rig.
Some P31 mobos and at times some G31 ones too, have no boot or stability issues with double-sided RAM populating all slots due to chipset limitation or a bad batch of mobos.