I'm not a programmer or expert user, so go easy on me please.

I have an old athlon machine that can generate ~1850 khash/s. I have a geforce 8500 gt on that machine, and used this to up production to ~2150 khash/s (says GPU 2150, so I know I'm using the -gpu). How can I get these 20,000 khash/sec that everyone else is? Is my athlon the bottleneck here, or can I just buy a nicer video card?