Take a look at the gtx 670, for $400 you can get a nice non reference model, like the asus direct cu($419), the gigabyte windforce oc($399), or the MSI gtx670 power edition ($389). All of these cards are pretty good, as they all have non reference coolers, and enhanced overclocking capabilities. The MSI power edition supports overvolting, so if you want to push a high overclock thats a pretty good choice, although MSI has recently been caught overvolting their cards by default.
The gtx670 is a nice card, and when its overclocked it can match or beat a gtx680.
Another option is the hd7970, which also can be found for about $399, and performs quite nicely, and if overclocked can really unleash its power.
Whats the power supply in your system?
Hope this helps!