I wouldn't suggest just giving up completely. I mean, there is or was a bad batch, not a completely corrupted version of a GPU. Have you tried reinstalling Windows? I know that absolutely does away with any driver conflictions. Also, EVGA has, IMHO, the best Customer Service I've ever seen. They will work with you, and if you have to go through several RMA's, might as well be through them.
Also, if you are getting tired of the 7900 cards that seem to be giving you trouble, you could also try an X1800XT(L), or heck, a 7800GT might do you fine (they are a bit pricey due to their discontinued state though). I'm just giving you options, I know you said somewhere that your PSU won't be able to handle an X1900. If you really want to wait a little longer for DX10 though, go for it. Best of luck.
EDIT: Sorry if I went over some points that were already discussed. I've slept since I was up to date on this thread and probably forgot some answers you might have already provided.
Also, if you are getting tired of the 7900 cards that seem to be giving you trouble, you could also try an X1800XT(L), or heck, a 7800GT might do you fine (they are a bit pricey due to their discontinued state though). I'm just giving you options, I know you said somewhere that your PSU won't be able to handle an X1900. If you really want to wait a little longer for DX10 though, go for it. Best of luck.
EDIT: Sorry if I went over some points that were already discussed. I've slept since I was up to date on this thread and probably forgot some answers you might have already provided.