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.
How is your BFG 7800GTX OC factory running @ 430/1200-
those are reference speeds for standard 7800GTX's.
sure, lets go for it, got plenty of time on my hands, lookin at addin liquid collin to it and i just revamped my cpu from 3.2 to 3.4 no problems and droped my mem mhz to 500 and lowered latency on it, so y not lets go for the vlotage mod
Improve cooling and Voltmod it. You should be able to get substantially higher clocks. More V will banish artifacts at higher clocks. Will you get to 600Mhz? probably not. 550 perhaps...
don't put too much voltage through. I killed my 9800Pro by running the core at 2V. You don't want to do that now, do ya... Just looks around and see what others are managing to run the card at (voltage-wise) with similar cooling setups, and try to stay a bit below that.
Raising my 9800Pro's GPU voltage from 1.7V(stock) to 2V only got an extra ~60Mhz or so, compared to stock voltage, so don't expect miracles....