If you'd like to start gaming, I expect you'll find that your 8600gt under-performs your expectations. However, your prices are out of line.
8800GS should run around $80
9600GT should run around $100
8800GT should run around $120
HD4850 should run around $150
These are after-rebate, good prices (you might need to keep your eye out). All of them will significantly outperform your 8600gt, and are worthwhile at their price points. But, you should buy a game or two first, and make sure that the 8600gt isn't good enough for your tastes.
The boys at the shop are wrong; a 9600gt destroys a 8600gt.