Did you configure the RAM in the BIOS, or just plug the modules in and rely on the Auto settings?
If you didn't change any settings manually in the BIOS then the board will have set the RAM to run at JEDEC specification- which is actually your RAM's spec (1.8v, 5-5-5-15, DDR2-800). When using four memory modules the voltage often needs a slight increase to maintain stability. Your RAM is guaranteed to 2.1v (extended voltage protection) to accomodate this.
When I say slight increase I'm talking 0.05v increments (i.e. 1.85, 1.9, 1.95, 2).
Since the board boots I would suggest downloading a program called
Memtest86+ to test the RAM for errors and stability.
The easiest way to run the program would be to click either
"Download-Auto installer for USB key" and install the .iso file onto a flash drive, or click "Download-Pre-Compiled Bootable .iso" and burn to CD.
Once done, insert USB/CD and restart the computer, jump into the BIOS and reset the boot order to either USB or CD. F10 to save and reboot. The program will load (refer to the Memtest86+ site screenshots), start the test and the program will run through 8 tests, which constitute a "pass". I would run 7-10 passes to ensure that the RAM is error free. This will take some time. For 8Gb you will be looking at around 50-60min per pass.
If the RAM checks out OK then stop the test, get back into the BIOS setup and raise the RAM voltage by an increment (say to approx 1.85v), change the boot order back to your harddrive, F10 to save and reboot. If the system gets into Windows and stays happy then all good. If it fails to load or is flaky once it has loaded, then raise the RAM voltage to 1.9v, F10 out and attempt to reload. repeat if necessary.
If the OS wont load with the voltage set at 2.0v then there is a possibility that the memory controller (750i SPP) on the board is either not working as it should or is receiving insufficient voltage. I would attempt to eliminate RAM error/voltage instability first, so...
At this point I think it's probably wise to check the RAM for errors first, followed by the RAM voltage bumps if needed (and a small one will probably be needed) to see if the RAM can be made stable.
Edit: I'm assuming that both 4Gb RAM kits are identical part numbers. Is this correct?