I switched out my motherboard and went from a geforce2mx 32mb to a geforceFX 5700 256mb and didn't format although the guy at the store said 'You're going to HALF to format, windows will crash...thats a gaurauntee' .......didn't happen to me, but yeah generally it'd probably be better to do a format. You're switching out two completely different companies cards sooo format would probably be a better bet. Why don't you just get the GeForce 6800 anyway, ATI sucks =)