Depends entirely what you are planning on using After Effects for, and whether the new versions offer anything you might need which aren't in the older version or are enhanced to speed up workflow.
Question is, do you have an Intel Mac or a PowerPC Mac? If you have an Intel Mac, you should either find a used copy of CS3, or go ahead and get CS4. Why? Performance is greatly improved for Intel Macs, since it's able to run natively. I tried running AE7 on an Intel Mac when I first got it, and it was dog slow.
If you have a PowerPC Mac, then you're fine with getting AE6.5. It'll run like it's supposed to...Natively. It's up to you to decide whether or not it's worth the investment. You can always get a cheap one, and buy an upgrade version later.