Of the two choices you offer, 2 8800GT's would offer the best performance.
The 9800GTX is nothing more than a reboxed 8800GTS 512 with slightly higher clocks.
If you are still in the research process of your new build, keep in mind ATI and nVidia will be releasing their new products in less than a month. nVidia's new GTX 280 is supposed to be faster than a 9800 GX2 for only a little more than you would need to spend on SLIed 8800 GT's (taking into account $ saved off a SLI motherboard).
Recommend you wait the extra month before sinking your $$. If you don't like what you see then, prices for the 8800GT are expected to drop.