What you should get depends on your current setup, which we know nothing about. We have no idea about your CPU, RAM, PSU, or even what resolution you play at. Had you bothered to actually give us info about your system, I/we could have given you advice.
I do know which cards are good for $150, perhaps it will help. You can certainly find an 8800GT for that amount, possibly even the 8800GTS. This is assuming of course you have a PCIe expansion slot. If your on AGP, your options are a lot less rosy. The best cards are the x1950pro, and the 3850. I have no idea how much the AGP versions of these cards are.