use single platter 500gb 7200 rpm drives for near maximum performance on the cheap. No platter based HDD will exceed sata 2 bandwidth for quite some time.
This is the one I usually recommend but newegg is out of stock on them. Too bad for $50...http://www.newegg.com/Product/Product.aspx?Item=N82E16822152181
After those I usually recommend the Western Digital RE4 500gb drives though they are 20-25 more. (not the RE-GP series)