I think macro is assuming you have a 5000 RPM drive, but I'm willing to bet it's 7200. Very few 5000's are made anymore. And if you do have a 5000, I haven't found that the latency is much different on a slower drive in gaming.
I would say if you're willing to spend, a 3870 is great, but if you're on a budget, the 3850 is good as well.
For dual channel to work, both sticks should be identical...so a pair of 1 GB sticks is preferred.
http://www.newegg.com/Product/Produc...82E16814102794 3850 $90
http://www.newegg.com/Product/Produc...9005&Tpk=650tx 650tx $95 (cyber monday sale has MIR)
http://www.newegg.com/Product/Produc...82E16820227189 $22