First, what you say of ATI and nvidia is all to true.
Many in these parts are jumping on ATIs bandwagon because this year nvidia isnt the cool thing to pick.
I've been ripped off by ATI also, but never by nvidia.
I trust the FX series to skyrocket with some driver revisions from nvidia later like all their other cards.
About your problem, like the other guys said something is causing it in your system most likely.
Were all those RMAs video cards? I'd check your power and see if you've got a ground also, you can get a tester for $2 at walmart, its a little yellow plug with a light on it.
You shoudl download some monitoring software and log the voltage lines on your PSU, see if there are any drops in any of the lines at any time.
I've had bad power supplies in the past do similar things.
If you do end up getting a new one (its not a bad thing to have an extra PSU around anyway), try an Enermax.
When you get your new one, try it in your roomies computer first, that way if it works there.. then doesnt work after its in yours, you can be sure that its not the cards being faulty themselves.
You could also pick one up at a retailer that does returns like best buy, try them and see if they do the same thing, maybe newegg dropped all theirs, who knows.
Athlon 1700+, Epox 8RDA (NForce2), Maxtor Diamondmax Plus 9 80GB 8MB cache, 2x256mb Crucial PC2100 in Dual DDR, Geforce 3, Audigy, Z560s, MX500