I personally would take the eVGA, mainly because I've been a loyal customer for my past three cards (7800GS, 7900GT, 7950GT KO), and their customer service has been fantastic. I had to RMA my 7900GT twice, and both times the RMA service was prompt, easy, and I ended up with a slightly nicer card than the one I had sent in.
You pay a premium for eVGA, but I think the overall experience you get more than makes up for it.
And I vote for the XFX because I've had several of their cards including my current 8800GT's and I've never had a problem let alone had to RMA one
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Forums are like a herd of performing elephants with diarrhoea -- massive, difficult to redirect, awe-inspiring, entertaining, and a source of mind-boggling amounts of excrement when you least expect it.
EVGA supports after-market cooling and has the step-up program, XFX has a lifetime warranty for you and anyone you sell the card to. Other then that I can't say I would feel bad about buying either unless the XFX one has that crappy cooler(not the reference nVidia cooler, the one XFX also uses on their 8800GS, it doesn't cool as well and is louder)
I am putting in my vote for eVGA. Their step-up program is what sold me on them. Since you are buying an 8800GT you can step-up after 90 days to the 8800GTS 512 if you end up needing more horsepower.
I have not had to RMA my 8800 (fingers crossed) and their warranty allows you to overclock to your hearts content. You can even remove the stock cooler and replace it with a third party one if you so desire. So long as their is no damage to the circuit board you can RMA it if it breaks.
I have had good experiences with the eVGA forums. I have not owned an XFX product so I do not know how their support is.
