X1900 XTX on ASUS A8N32-SLI Deluxe

I don't see why it wouldn't work. I believe you are thinking it won't because the card is ATI, while the chipset is Nvidia. If this is the case, you're wrong. As long as the motherboard has a PCIe 16x slot the card will work, it doesn't matter who made the card.

There is one other thing you might be missing. Both the 7900GT and x1900XTX are power hungry cards. If your psu is small/budget, that might account for the issues you're having with the 7900GT. These issues won't go away by switching to a different video card. (that happens to use even more power then the 7900GT.) If you have a 350-400W powersupply, or one made by a fly by night company, that could be the source of your problems. You should consider upgrading if this is the case.

Lastly, I've never RMA'd from Brasil but its not a bad process here in the states.

I have an A8N32-SLI Deluxe and have run a single X1900 XTX for a few months now with no problem. The one I have is from Sapphire and has watercooling, but any X1900 XTX should work without problem.

Its recommended that you have a psu of 550 wt or greater, but a 500 wt should do if you're not running a lot of other hardware. I haven't heard of SevenTeam, so don't know if that is a good one or not. If you have problems with the psu, Antec, Enermax, Hyper, and Seasonic all make good psu's.