Hmm, a e6850 (300$ on Newegg) might be worth it if you also overclock it to 3.3-3.5ghz. That'll give you the best performance possible today, except in applications that support quad core( which are still very few ). 8xxx line is mainly for DX10 as Fyron says, although there is some performance increase too.
Here's a user-made test you can have a look at, comparing your card to the 8800GTS 320mb.
http://www.bjorn3d.com/forum/showthread.php?t=13355