Quad SLI will cost you $2000 at a minium. The GPU's run $800 a piece at a minimum(you need two, they're dual core) and then the PSU needs to be at least of 850watts and very solid, that's $300 if your lucky, more like $400. Then you need the best CPU to kill off any chances of bottlenecking, so that's $900 at a mimimum, either an FX or EE. Then you need 2x1Gb of fast memory again to prevent bottlenecking and to be prepared for newer games. That's most likely $300 at a minimum for good memory with low timings.
Hmm that's around $3000-$3500 and excludes the motherboard, sound card, case, aftermarketing cooling, hard drive(s), monitor ... ect.
ATI does have a plan to respond to Quad-SLI, I read it somewhere. But I don't expect it to come to close to Quad-SLI's performance.
But realistically, a single 7900GT, X1900XT, 7900GTX or 7900GTX will take any game up to 1600x1200 with great graphic quality. SLI isn't worth it yet, unless your running monitors that use resolutions greater than that, with loads of AF and AA enabled on multiple monitors.