the x1 slot is 2.0 from the x58
bios with pciE adjustments... idk...
modded board, i doubt it, i dont think any br boards were ever shipped pre-modded, only gti and Fb and Fb gti...

you dont need an adapter, you just have to open the end of the x1 slot carefully, then you can insert a x4 x8 or x16 card in it

ppu... well last i checked they were way slower than even a dedicated 9400gt... ouch...
so that kinda makes sense i guess...
i didnt know they ever made pciE x1 ppu cards...

def wait for vgas, once the 5750 is out for a while prices will drop more...
are you willing to spend 500+$ on gt300?
what display do you have an what res do you game in?