Found my own answer in the manual.  Out of the 3 PCI-E slots only two of them are 2.0.  Not to mention when I looked in the case there is no way I could possibly fit a third between the 2 GTX 280's already in there...

The 730 does have an nvidia 790i mobo, so it should support tri-sli.  Not sure where one would get a connector or if the psu that it ships with it is enough for 3 cards.

 

Don't know if dell 'crippled' the 730 to only support 2 video cards.

The 790i chipset supports only two PCIe 2.0 slots with one additional PCIe 1.0 slot. Thus Dell's MoBo is consistent. Dell's 790i supports three-way SLI.

 

PCIe 2.0 was developed because PCIe 1.0 supplies only 75 watts of power. As a result, more powerful GPUs needed additional external power supplied by a power connector, either a 6 pin 75w connector or an 8 pin 150w connector directly from the PSU. PCIe 2.0 slots supply 150 watts. Of course, after 2.0 was introduced, we still end up needing even more power . . .

 

PCIe 2.0 also provides double the bandwidth but current cards have yet to max out the bandwidth, however they very well may within the next year or so. GPUs that are 2.0 compliant are also backwards compatible and will run in PCIe 1.0 slots. They may need more external power however.

 

Physically getting three of one's favorite GPUs onto a 790i MoBo is yet another matter entirely, as the OP notes. :)

Thank you for agreat explanation, it answered the questiuons I had about the difference in PCIe 1.0 vs. 2.0