the 4850 and 8800gt are not really even. they are priced according to performance right now. i'd splurge for the 4850 as it is consistently faster than the 9800gtx and can be found for cheaper.
oh and if it's not been made clear enough already, don't bother with sli on a 7600gt. honestly, in less you're playing games at 1920 or above resolution with aa/af on then there is no need for a dual card set up at this time. ya, you can go from 60fps avg to 90 fps avg but your bottom fps will still be the same as most games at this point are limited on the bottom end either by cpu or memory bandwidth/capacity of the vid card.