Yeah if you can afford the 6600GT then that'll offer you awesome gameplay. It'll actually be more than you need really and if you're really worried about PCI-E and all that (which really I think some people are jumping the gun on that) then just wait a year until you upgrade
I think AMD64, SLI, and PCI-E are overrated right now. Common case of 'OH NEW TECHNOLOGY GOTTA HAVE IT' when it's not even fully utilized or standardized yet and won't be for months to come.
A good OC'd socket A board with good memory, video card can definitely hold it's own to some AMD64 CPUs....about up to the 3500+ and after that, the GHz of the AMD64's will just outperform the GHz able to be achieved by the OC'd processor.
If you can get your CPU to 2.5-2.6 it will be more powerful than the lower AMD64 CPUs.
Also yeah I hear good things about DFI's overclockability although I haven't fully looked into it.
I think mainly it'll allows for a higher overal FSB, but if you're doing that, then you're OC'ing your memory as well which is more difficult than OC'ing the CPU IMO