If your psu has 1x6pin pci-e cable, then the answer is clear for under $80: XFX HD-675X-ZMFR Radeon HD 6750 1GB 128-bit GDDR5: http://www.newegg.com/Product/Product.aspx?Item=N82E16814150572
$74.99 after rebate.
edit: also for $9.99 more than your budget at $89.99 XFX6770: http://www.newegg.com/Product/Product.aspx?Item=N82E16814150540
This shouldnt matter when you are buying a gpu, but its a nice bonus since its free, both cards offer free games:
6750: + Free Gift: XFX Gift - Game Coupon with game key for full version S.T.A.L.K.E.R: Call of Pripyat (value:$29.99)
6770: + Free Gift: XFX Gift - Dirt3 Game Coupon (value:$49.99)