5850 --> 7870 or 7950?

[cmdrdredd]

Doubt it. CrossFireX and SLI are limited by the weakest link in the chain.

SLI won't work unless you have identical cards with identical memory amounts.

For example a 4GB GTX 670 won't SLI with a 2Gb GTX 670. Likewise a GTX 680 cannot work with GTX 670.