For improving performance, it doesn't hurt to have a better processor, or RAM, or video card, but...that stuff costs money.
If you already have a good enough video card to run the game, there are a few things you can do. These are not guaranteed to work since I do not know what game you are playing, but most games with the following features will see improvement. There's surely more advice, but this is what I can think of off the top of my head.
1: If the game has the option, disable shadows. This seems to be an almost-universal truth amongst games that have them. Shadows cause lag, and serve little purpose.
2: Anti-aliasing can be turned down/off as well. Yeah, the game isn't going to look pretty, but it's going to run smoother.
3: Internet connection. Broadband/High-speed-otherwise is the way to go. Dial-up just can't handle online games, at least nȯne that I've played.
4: Draw distances should be turned down. The less your computer has to load in advance, and keep loaded, the better.
5: In-game Items should be kept to a minimum. It may seem imperitive in some RPGs to horde all the items you can and sell them en masse to vendors later, but the more items you have the game keeping track of, the more resources it's likely to need, and so the slower it's going to run.
6: Useless/background programs. Anything you have running in the background uses resources. Anything. That quicktime or winamp agent? Kill it. Daemon tools? Kill it. Anything you don't need, and which isn't going to shut your computer down, kill it. Internet browsers, folders you're not using, media players, USB Device/PDA Helpers, etc.