Well its 20k cheaper than the SLI gtx.
for sli 7600GT?? is that card still alive? THats even more impractical since its already outdated.
as for the topic
8800gtx is currently the fastest card out there no question. and
2900xt is comparable to 8800gts 640mb and even faster since the improvement of the catalyst drivers.
and to choose between them the 2900xt is top of the line VC from ATI plus it has advantage on HD movies.
HD2900XT is at this time placed nicely in-between 8800GTS 640 and 8800GTX in terms of performance.
http://www.legionhardware.com/document.php?id=645&p=1