It's risky yes. I was doing the same thing but with a 8800GT and i was getting crap performance and constant shutdowns. You'd probably get better performance out of it if you get the minimum specs, that's why they're there :-D
Yeah I do, I have 3 gigs of RAM, 8600 GTS and a pentium D at 2.80! I need to get a new power supply, at least 500W and a new case!