You can make your computer faster for gaming in a few different ways. Here are a few tips for improving performance without buying a new computer:

• Decrease game settings - Some games come installed with top of the line graphics enabled. Unfortunately, older machines will struggle with these high end features turned on. So, the easiest way to improve performance is to decrease the video settings.
• Close every application - While it won't help you pump out more polygons, closing every application and freeing up the processor for just the game will speed things along, and every bit of performance helps, even if it's marginal
• Disable high end graphics - Besides game settings, nVidia and ATI cards come with control panel applets that can disable high end features on the cards.
• Get more RAM - Sometimes there's nothing you can do in software, and you need to resort to physical upgrades, and RAM is the easiest (and cheapest) way to improve performance - just buy a new stick (or two), stick it in, and you're ready to go.
• Get a new Video Card - Of course, sometimes there re games that just use features your video card is simply incapable of rendering, forcing the game to render video on the processor. When your video card is this outdated, it's time to buy a new one.

So, there are your options: you can decrease software features, or upgrade hardware features. Eventually, though, you're just putting off buying a new set of hardware.