So I want to upgrade my GPU's on my old rig. I have a very nice Asus Rampage Extreme LGA775, Q9550s, 2 x ASUS HD4870 Dark Knight 1GB cards, 8gb OCZ Reaper, Kingwin mach1 1220Watt, 2 x 80GB Velociraptors in raid0, 2 x 750GB WD Blues in raid1. This has been a great system and meets my needs. However I figure with time even an old rig can benefit from new GPU's. Heck I put an HIS HD4670iceq in my old Abit IC7 Max3 to replace an nVidia 7800GS board and my son still uses it and plays UT2004 (awesome with that card). So my question is this:

If you have an older system and the sky is the limit for buying a new GPU or two what should I put in based on the fact that it's PCIe 2.0? Would I benefit from a one or two HD7970's or HD7990's? or would I be better off from performance stand point with something less?

Thanks Rodney, you rock as usual!

EDIT: Obviously this was supposed to be answered by Rodney and his expert knowledge which I've sought and valued greatly over the years, but the moderator moved it, gee thanks. This could have been a video review about getting a little more life out of an older system, which is what I wanted and may help others. I hope my second attempt isn't met with such disdain.