Wait until there are more benchies and drivers that support quad SLi unless you want some expensive paperweights or something to beat small children and animals with.
The GX2's may end up being a good choice but with a price tag of $1200 for two it's very expensive. From what I've seen 20k should be doable with a Q6600 and two G92 GTS' but it would be best to try and get a G0 Q6600 because you're going to need a pretty big OC on it. For now wait to see if quad SLi = fail.
If you don't go with GX2s I'd go with 9800GTXs rather than 8800GTS' mainly because the GTX will have more core voltage and faster memory. The GTX will probably OC better, fueling this I remember reading something somewhere that someone had vmodded one and got 900MHz core on air. (Source