This is of the CM 690 II Advanced with a GTX 260 in it. Pic That card is 10.5" long, and apparently the same length as the 5870 Vapor X. http://www.legitreviews.com/article/1120/1/ Remember that the PCI-E connectors will be coming much more from the front of the case, altho I would have thought there'd still be plenty of room.
Personally I would save myself £15 and get the G.Skill, but nothing wrong with the OCZ.
Here's a review of the Antec Earthwatts 750.
I think it will certainly be able to provide enough power for two 5870s and an overclocked i5.