Which SSDs are compatible with my old Macbook Pro?
Everymac.com is as close to a comprehensive source of system information as you'll find. In the case of your Mac, you can see it has a 1.5Gb/s SATA interface. So you won't be able to get the full potential sequential transfer speeds of a recent SSD, but keep in mind that the most noticeable benefit of an SSD comes from the random access speeds, which are less likely to be bottlenecked by the SATA interface.

It's also worth considering that SSD prices have come down a lot lately, so you're less likely to save much by getting an older model. I'd recommend looking at the Samsung 830 series or the Crucial m4. Both have excellent performance and good reliability.