SLi is utterly pointless, its more faff than its worth - its not 100% efficient and its more power hungry, just get an 8800GTS and be happy, you cal always SLi up an 8800GTS at a later date.
Usually for the price of 2 lower end video cards in sli, you can buy one higher end card for the price which will have better performance. So sli is really only worth it if you are going for two high end cards like the 8800 series. Correct me if i am wrong.
I'm doing a custom (very custom) case build on this bad boy too. Had all the parts in it, figured out where all the wiring is going, then took it all apart. I'll be putting the HD's in a custom base under the main case. Eventually this will all be water cooled as well.
I definitely recommed the 8800GTS- especially if you have enough space. I took the HD racks out of my case (drilled out the rivets) and put the power supply there instead (yes, I'm reworking venting of the case too.)