Graphics performance limited by CPU?

Thanks for your input, first I was going to stabilize my CPU clock to about 4ghz (FSB470/locked 8.5x) to maximize my cpu speed to keep up better with the sli's doubled-bus speed with two cards serving as one and such.

Issues arose:
The cards came with a CD with the 320.18 drivers which were previously known as he "card killers". I went to download/install Nvidia's newest-I think it was the 320.48 which kept failing to install on "Installng graphics driver". I Googled others' experiences, saw that a fresh install (removing all existing NVUDIA programs first) worked for some. I looked further into what could be th cause and found the ASUS P5Q PRO was apparently an ATI enthusiast board, meaning lack of SLI support. "Okay," I thought, "Life goes on, a backpack GTX 760 still works as a PhysX card, right?" After accepting this I removed all NVIDIA software & drivers to reinstall was making sure everything was updated, then rebooted the system.

After the POST message, the HDD was detected but returned the error Error: Windows failed to start. A recent hardware or software change might have caused the issue." So I remove the SLI connector and reboot. Nope. I remove one o the two GTX 760 cards. Same issue. Maybe the software/driver installing I just did removed the bootable flag? I reboot into my trusty Gentoo live CD for some commandline FDISKing to get an error when loading it in related to something not connecting.
I boot in to a second bigger HDD I hve been using as a slower backup and backed up the documents/downloads/etc to reformat my faster primary drive. The Win7 insall CD console startup repair just clocked to no avail. Reinstalled Win 7 and it seemed to boot into it operating normal after install. I ran Winows Update, set a restore point, and proceeded to run the NVIDIA 320.49 fresh this time. Upon reboot, the same issue- Windows failed to start.. Pop in an Ubuntu Live CD this time, but now I don't get POST or even BIOS access. This system appeared dead.

Steps taken so far in rulng out hardware issues:
-Booted an older second motherboard to BIOS with PSU, do not have another compatible PSU to test with the failed system.
-Attempted boot with older graphics card (GTX 260) to no avail. I know the card works but older MBs I have don't detect it even powered in their PCIe slots. GTX260 requires 500W minimum but other PSUs I have around are 250W, 300W, 350W, besides my primary 750W. The 750W does not have compatible 4-pin plug to accurately test it with available older boards.
-HDD is detected when connected to other old systems, haven't had it boot anywhere yet after the aforementioned issue.

In addition to that which is noted above, I am looking into buying new parts for an nForce-based system. I welcome any advice on what you have seen or used/have which could harness the 2SLI 760 comfortably. In particular, MB, CPU, RAM/timings are my dire concerns, though again, I'm open minded.

Thanks in advance!

Thanks for responding - I should have mentioned that since I started overclocking resetting the bios to default has become my first response to troubleshoot system instability. At this point when I hit the power button, the fans start but the monitors don't turn on. Not even seeing the bios to make changes.. I wouldn't think the CMOS died but would that be possible? It seems too convenient (probably not the right word) that my system has been headdesking since the new graphics cards. Tough to identify an issue when its CMOS and that is never considered. First the HDD stops booting, then Live CDs become erroneous, now the graphics do not display on monitors. I think I followed a OC tip to manually key in the PCI voltage or something to 100 to be safe. Could this happen if the setting was undone when I defaulted BIOS and forgot to set PCI bus to 100 hard keyed? I'd usually OC within stock voltage, just briefly went up to 1.325 to run a benchmark. Also, about PLL and GTL settings, I kept PLL at 1.54 and GTL at 0.61x, riskiest settings I've changed so far.