Drivers rarely, but can, cause this very problem. Often the system would give an error normally but the reboot happens so fast you never see it.
Cleaning and reloading the drivers never hurts. It's not likely the problem, but not impossible.
I agree with the others in that it's more likely the ram.
Some things to try:
Without looking at your board it's possible that it might not like the particular format in which you've placed your ram.
"Maximum" ram capabilities of motherboards can be misleading.
Check to see if your specific ram configuration is ok, there should be a matrix online or in the manual for it.
For example, many times the "maximum" capacity can only be reached by slower memory, or single-sided memory only.
My board for example, which is an nforce4/amd, can go 4GB max, but, only at 4x 1gb 266mhz with single sided memory. The board may well post with other configurations (and has) but will eventually fail randomly. I know, because I've tested this very thing for other people btw.
At any rate.
Easiest step is to take out two of the dimms and see if that helps, or better yet go down to one. If that doesn't help, reduce the memory timings and speed. If that DOES help, try going back to the stock timings but with slightly higher voltage.
It will be a pain, sure, but that's how it goes with "random rebooting" type stuff that isn't obviously heat.