This article will show you $300 options. Then use the table in the back to pick your AMD card. http://www.tomshardware.com/reviews/gaming-graphics-card-review,3107.html
Given you only want amd the choices are limited
HD7850 = $240
HD6950 = $250
HD7870 = $300
HD7950 = $340
Or hit ebay hunting for a 5970 or used 6990
EDIT: Opps skipped the disk
I'd get a high performance disk becuase I got a pair of green ones then really noticed the boot time difference on the PCs.
WD Black 1.5 TB is a fast drive = $160. I bought this same drive for $90 prior to the Thailand flooding. If you can wait a few months HDD prices should fall closer to $100.
That leaves $150-$200 for video
HD 7850 - $240
HD 6870 - $165
I'd get HD 6870 and wait for a sale on WD black 1.5TB to meet $300 budget.
However, you should look at framerates of the games you play and see if you would be happier spending less on video and getting worse frame rates, or spending more on video and stepping up to 7850.
Or hit ebay for a HD5850