Getting 8GB of OCZ PC6400 DDR2 to work with my XFX 750i SLI motherboar

Ok. I have an XFX 750i Motherboard model MB750IP29 and i bought 4 @GB sticks of OCZ PC2 6400 Part# OCZ2N800SR4GK and the timings listed on the heat spreader is 5-4-4 @ 2.1v. I can 2 sticks in slots 1&3 (dual channel mode) just fine. I have overvolted the DIMM slots through BIOS to 2.1 from the 1.8 factory default voltage. If I add a third stick to slot two the motherboard boots through BIOS and BSODs at the windows 7 startup screen. I can adjust memory timings manually through BIOS but have no clue on earth what each one means or does so I have left them all at auto. If I add all 4 sticks it just sticks on a black screen during initial boot and wont even begin to load windows. I have tried overvolting the DIMM slots to 2.3 because I read in another article in these forums that might solve a similar issue someone was having. I paid $200 for this RAM and would greatly appreciate the anyone that could help me to get it running. I'm somewhat new to the intricacies of memory timings and BIOS adjustments but i DO know that even with all 4 GB installed the motherboard is still only going to recognize 7GB. I'm ok with that....but as I've said...I would GREATLY appreciate any help!!!!!! thx