A 3+ year old machine isn't gonna cut it if you have thousands of raw images that need to be worked on.
If you end up liking Lightroom, you'll end up buying new hardware anyway. It might be faster than Aperture, but not a whole lot faster. (Actually, it doesn't process raw files any faster; it's simply using a low resolution JPEG proxy when you view images.)
If you want to see how slow Lightroom's raw conversions really are (i.e., no faster than Aperture), just go into Develop mode and click (and hold) on a slider. The image will immediately degrade to a jaggy low-res image as you drag the slider around and will only update to a high-resolution image again after 5-15 seconds. And this is on my dual 2 GHz G5 PowerMac.