The SN26P was socket 939 wasnt it? also had SLI if i remember rightly (basically the upgrade from the SN25P)
If I'm rememberin it right then the best cpu you could get would probably be somethin like an FX60, Opteron dual core or a high end X2 dual core or somethin (I'm thinkin Opteron 185, X2 5200+ kinda thing)
As for graphics, i dunno? lol Didn't it have a special heatpipe for the GPU's? if so then you won't be limited to single slot cooler cards and it would just be a matter of length more than anything. Shuttle PSU's are very high quality and tend to run basically everythin you throw at them so dont worry about the power requirements of high end GPU's too much