Hey Butch,
hard to find direct IGP/non-IGP chipset comparisons, but it looks like if you use a video card, and not the IGP graphics, there is not a lot of difference:
Gary Key wrote last year:
http://forums.anandtech.com/me...id=29&threadid=2008282
In answer to your concern, exclude any overclocking aspects of the boards, the difference between the G965 and P965 is about 2% on average without a discrete video card solution installed in the G965. The difference with one installed is negligible if all other settings are equal. The issue is that most G965 boards do not offer extensive memory controller options and so the ability to tweak the memory is not available and the memory subtimings are usually run a lot looser.
That's P965 v G965, so a bit out of date....but, P35/P45 and IGP versions are descendents. So, I would guess same would hold, non-igp version would be slightly (a few %) faster, but with better memory tweaking options, etc.
Recommendation for you? Since you have the nice high performance video card, get the P45 chipset: better memory control, and, I think better drivers than the G chipset versions. (Some of the GXX chipset drivers are sort of flaky from what I understand....the P45 drivers are cleaner, better for your needs I think.)
HTH
NXIL