Upgrade my GPU or my CPU (Athlon 6000+ / 4850)

I have around $250 for an upgrade to my current box. I am still trying to feel out which would be better to upgrade. Right now my system specs are as follows:

Athlon X2 6000+ at 3.1
8GB at 800mhz
VisionTek HD 4850 512mb
HD 160gb at 7500
790FX msi board (supports AM3 with flashed BIOS)
powersupply 500w (which restricts my GPU choices I assume)
Monitors: A 1080 primary and a 1280x1024

I am really torn on weather or not I should get a new Card like a 5770, 5850. Or a new CPU P x4 955, 965.

I was also curious about the drops in price that will occur over the next 6 months.