I'd hedge my bets on it just not working. The 4850 will make short work of your current graphics card. Also I hope your CPU is good enough to let all of the magic out of the card, so to speak.
I figure your right. Shortly after posting i came across the CF combo chart
which i'm taking as "if it is not on this table it won't work!"
As for my CPU i would hope that my intel core 2 Quad @ 2.66GHz would suffice. Along with its 6gb or RAM.