Personally as far as what you have given goes, I would go with 2/3 3870s. As a general rule 3870s in CF do as well as 8800GTs in SLI but for less cash. And also you could keep the same motherboard.
Currently the 8800GTX really isn't worth it. There is supposed to be a new Nvidia card coming out some time soon. (I'm not sure when, some one around here probably does.)
Also I would look for a higher resolution monitor. (1650x1050 native in a 24" isn't that great.)