Reasoning: At lower resolutions (1280x1024) the CPU is a bottleneck for SLI right? so would haveing a second processor boost frame rates? (one core is dedicated to the SLI thread, that is, if SLI works like that)

If this question has been answered please point me to the tread. If I'm wrong about the bottleneck, please correct me, because I am honestly not sure what I'm talking about. Otherwise it would be cool if people posted benchmark numbers, framerates, etc. along with system information so we can get comparisons going.

Thanks

The latest Nvidia drivers increase performace on dual core systems by 10-15%.

compared to what? dual core with the previous drivers? or a single core of comparable performance? (though that doesn't really make sense to compare a single core and a dual core in this way)

I guess my bottom line question is: will I get faster frame rates on an SLI configuration with an Athlon 64 4000+ or and Athlon X2 3800+/4200+? (those processosors are comparable right?)

Most of the time, dual core does nothing for games. Games are very much single threaded. As Ned pointed out, however, with the newest nvidia drivers, and SLI, the second cpu is actually being used, to some advantage. Numbers up to 20% more frames have been reported.
I take that with a grain of salt, as the gfx companies have ways of making things seem like they work better than they do. Still, if you cant notice anything, but faster framerates, it looks good to me.