Anything I add here is only a footnote to AigoMorla's post.
By itself, a Peltier cooler can not be effective beyond 65W of thermal power -- given the size in square-centimeters of today's processors and GPUs. This is the reason that the hybrid Peltier-heatpipe coolers such as that AigoMorla mentioned appeared on the market just when the Core-2-Duo processors appeared. The C2D's have a thermal wattage TDP of 65W.
Some of my friends here are going to say "Oh, no! There he goes again!" when I suggest that some sort of ducting mod might resolve your VGA cooling problem (if I read your initial post correctly). At worst, you would spend less than $20 for some foam-art-board, some Zapo "foam-safe" cyano-acrylate glue and an Xacto-knife -- and the cooling effect might still leave you unsatisfied. But that's a lot better than a high utility bill (recurring cost) and the investment in the Peltier PSU -- or the trouble you'd go to with integrating Peltier with your existing PSU to no avail but complete frustration.
My partial odyssey in a "ducting mod" project can be found in a post I started back in June or July:
Motherboard Ducting from the Bonzai-Duck-(t)-ster-man
The part that might interest you would be that describing the extension of the motherboard duct to cover the VGA card. One tentative modification (and use of a 92mm fan) reduced the VGA load temperature on my 8800 GTS (640mb) to 42C degrees.
I've since modified the duct to replace the 92mm "puller" fan with an 80x15mm "pusher" fan. Without the duct itself, my last experiment showed the VGA idle temperature around 51C (with no VGA fan) -- a load of 51C with some mild gaming, and a peak temperature of 57C under extensive 3DMark06 benchtesting. I replaced the original cooler with a TR HR-03-Plus to get this result. The peak temperature with the original cooler was 70+C.
With the 80x15mm fan in conjunction with the duct, the idle VGA temperature is around 43C @ 75F room-ambient, and peaks at 51C (load).
You didn't say which after-market VGA cooler you chose. I can surmise the possibility that you didn't choose the one with the highest-performance and lowest thermal resistance, but I have no basis to confirm it.
OTherwise, Aigo and others here have given you some good advice. And -- yes-- why over-clock a 7900 card when you can get a DirectX-10-ready card -- if only an 8600 -- and get at least that performance without over-clocking the GPU and graphics memory? An 8800 GTX card almost matches two 7900 cards in SLI for performance.