Radeon 5570 for older desktops, especially Dells. Which has crappy PSUs and usually no space for a big card.
Runs WoT on high at ~30-40 FPS.
Runs AH maxed except shadows at 8120 (Or whatever it is).
If your PC can't run a 5570, then there is literally no point in trying to upgrade it. You'd be better off buying a new PC.
5570 also uses ~10-20Watts, which runs on my crappy 220W PSU, even when OCed.
Very cheap, and not that bad either.