First off, why are you setting your target being the XT frequencies?
Nvidia cards can produce much better results than an ATI based solution at lower clock speeds. So if you want XT preformance, your not going to need those clocks.
Two, unless you have Liquid Nitrogen handy (doubtful) your not going to
push that GTX to default XT clocks (625/1500)
On the mem, hell yes you could make 1.5. But there's no way in hell your going to push that thing 195mhz over core, even on good water systems. Sorry but that isnt gonna happen. Maybe 500, give or take 15-20 mhz. But thats about all your gonna see.
EDIT:
How is your BFG 7800GTX OC factory running @ 430/1200-
those are reference speeds for standard 7800GTX's.