Aye, as I came to understand much later lol . . .
. . . the mobo was "bad", never did SLI or perform as expected. That mobo was eventually quietly recalled/replaced, an option only to those who happened to be following the Dell forum threads at the time. I was very disappointed in that approach by Dell, a company I otherwise respect and often (still) recommend when the right users/uses are involved.
. . . and there was over $2,000 tied up in the extreme quad plus the two then top-of-the-line graphics cards. Not exactly value purchases lol.
That 710 is now my #3 game box, now with a different single video card. If it were not a BTX, I'd still be using the humongus 50 lb case lol. Eventually, I'll probably use the PSU as a spare.