if I had the $550-600 I'd get the 9800GX2, but considering many sites online now are offering the 8800GTX for the $350-400 range, price wise its the better deal...
but if you have the money go for the 9800GX2, just be sure you have a PSU that can handle its power requirement AND, by all means make sure you have plenty of cooling to keep air flowing to that card (from what I've been reading, unlike prior GX2 cards, this one has both cards facing inward to each other to a common cooling element, so the beasty gets Extremely warm...)
but the 9800GX2 should offer much more performance than the 8800GTX, as its using 2x G92 core chips if I remember right & offers basically close to or same as 2 GTX's...
but if I were in your shoes, I'd hold out & see what the newer "normal" 9800's offer in price & peformance... there is supposed to be a 9800GTX out soon as well, lesser performance than the GX2 of course, but should be able to clock to higher speeds than a single card in the GX2 can, from what I read. (also a benefit if you don't have an SLI enabled Mobo, which I think is/was still a requirement for the GX2 like the old 7950 was)
hardware
prices 
