Dug, I hope I can shed some light on which GPU to get.
See my m170 specs below. The 6800 Go Ultra is nothing more than a PCIe 6800GS. I know the difference between AGP & PCIe 6800GS, so if get an AGP GS to run as fast or faster than the PCIe (which is safe to assume) here's the lowdown.
My desktop used to run a 6600GT (PCIe) with a 3.0ghz P4c & 1.5ghz DDR2 memory. Comparisons are not exactly the same, but both CPUs are very close runnig games about the same according to various websites I've read.
Last month I upgraded to an XFX 7600GT (590/1600). In the games that I run regularly (COD2, HL2 & Epi 1, Quake 4, FEAR demo, Chronicles of Riddick), both desktop & m170 run the same resolutions, all settings on full, 2xAA & 8-16AF, and all games run about the same speed. No benchmarks for you, but just playing the games they're pretty much the same. Actually, I believe the 6800 Go Ultra/6800GS can handle a little more AA than the 7600GT can.
Whichever way you go, you will notice a substantial speed difference.