Maximum PC

I have just built a system.
SPECS:
Xeon quad core (LGA1366) engineering sample (70$ on ebay for a new one from a good seller)
4GB RAM (Kingston HyperX Blu, will upgrade soon)
500GB Seagate hard drive
650W PSU (Cooler Master)
HAF 912 case
Zotac GT 520

What GPU should I upgrade to? I would like to get it at the NCIX yearly half off sale, and want to spend no more than 160$. I know this is a bit premature, but at this point, I'm just looking for ideas. Should I get a 600 series GPU, or use an older but cheaper 500 series one? Should I go to AMD instead?

The obvious suggestion would be a Radeon 6800 series card, but I don't know anything about suppliers.

Edit; Did you say half off? Get a 7800 (when it comes out).

NCIX half off sale usually has clearance items or lower/mid grade parts for sale; highly doubt any 7k series gpu's would be included in on the sale. Most likely it'll include the older 6k and the 400/500 series from nvidia.

Honestly shop around, I saw an 560 vanilla for $130 that was a pretty good deal; not to mention the 560 ti's were hitting below the $200 price point a few times. Maybe see when those hit the 150 mark or so.