Precision 690 SLI?

If I'm not mistaken you need a PCIe riser that splits the slot into 2 PCIe 16x slots (not sure if this will be 2 x 16 PCIe lanes or 2 x 8, but I suspect the latter).

Also, I believe the SLI is limited to the Nvidia Quadro range, not GeForce series cards (as this is a CAD/CAM system, not a gaming system). I'm pretty sure it's an Nvidia driver limitation.

Probably if you call your Dell salesrep or Dell spare parts they can probably find it.

Judging from section 5 here if it raises the slots like it looks like it might, you may need some special bracket too for allowing the videocards to secure to the chassis. I think it's the little piece you see here.

I'm not sure if anything else may be needed (powerlead splitter or so)... I'm not that familiar with the 690 (was interested in it at one point, which is why I know a few things about them, but never actually ended up buying one).

I'm not sure that Sli will work unless you order the PC with the option. Not an answer, but here is someone else who tried

http://www.dellcommunity.com/supportforums/board/message?message.uid=19018263#U19018263

I know there are a few differences in the different chassis of the 690's, Ive had two, and they very from the 750w to the 1kw options (extra space on the back for a memory fan on the latter) but it seems that if you can actually find the parts (good luck, might have to get a barebones chassis just to get it off flebay) that it may be possible