here's a link that might help you...and yes the 7600GT is better...(it's got a whole new line of technology so it'll obviously be better.) you shouldn't have any problem using a 7600GT w/ BF2 even @ mid-high settings.
but that 7800 isn't a bad deal either.
here's the data from it:
the 6800Gt isn't in 3dmark06 but it wouldn't keep up so well cuz '06 uses '06 technology in it so the 7600 and 7800 will pull ahead in it.
download 3dmark and that'd give you a decent idea of what the upgrade will give you.