In geekbench tests, the original macbook air (1.6GHz C2D) whups the pants off of all G5 systems except the dual2.7 and the dualdual2.5. This test is admittedly only of CPU and ram speed performance, and isn't necessarily indicative of the fact that even though there are no 64-bit apps for PPC macs, you could hypothetically cram sixteen gigs of ram into one of those faster G5s, or an nVidia GeFORCE 7800GT or whatever.
BTW, how has the ram swapping/testing been going?
After having read through this thread that's unfortunately the only thing I can think of in terms of what may be causing the system to be unstable, unless it really is just a faulty or mis-seated processor or other logic board component.
