Well, X1800XL are going for about $320 at the egg, and the crossfire master card is about $530-ish. So a $850 budget in NVidia land buys two factory overclocked 7900GTs, with about $150 left over for aftermarket cooling and new games.
I'd say the 7900GT x 2 is the closest competitor to your rig pricewise, even though it's significantly better performance wise. And if your friend moves fast, he could get them today.
7600GT x 2 SLI is not a very cost effective combination at the moment, even though it'd be pretty competitive with your machine. A single 7900GT is a better way to go for the same amount of money.
Also, if your friend gets 7900GTs from eVGA, there is a non-zero possibility that he could upgrade them to 7900GTXes within 90 days and still fit into the $850 budget.