I don't think you can install AE 5.5 on MacIntel hardware. You will have to upgrade.
And i forgot to say that AFAIK, multicore support was added in AE7, so even if you manage to run 5.5 on a dual/quad/octo core, it will only use 1.
If 5.5 runs at all it will have to run under Rosetta. If you want to give that a try you have to open up the Info palette for AE and check the box that says something like Run under Rosetta Emulation.
I'd be a little surprised if it would even install.
And i forgot to say that AFAIK, multicore support was added in AE7
There was some degree of multiprocessor optimization for some things in older versions, but the simultaneous frame rendering feature AE uses now debuted in CS3 (with improvements in CS4).
I don't think 5.5 will install in a contemporary Intel-based Mac. And in such an unlikely case, running it under Rosetta emulation would be a really, really bad idea. Bear in mind that the upgrade cost for AE is a fraction of the retail price for any of those Mac Pros.
Can you tell me if either of these computers work with After Effect 5.5?
The question is the wrong way round: Would AE 5.5 work on these systems? As the others said - most likely not. Even if the installer will extract everything or you simply use your already extracted backup and move it onto the new machine (which works with those old versions), you'll have to run it in emulation. Performance will still be okay, but you may encounter a million ill side effects. Most critically, you will have issues with Quicktime, as even Apple do no longer guarantee legacy compatibility. Furthermore, you may get annoying display oddities plus be unable to safe presets, prefs and stuff, because the old version may not be able to correctly handle different directory names and such. There's a lot of question marks in it. Of course you seem to be on a very tight budget, but I think you haven't really thought through your plan and what consequences it may have. As a minor, I would try this on a friend's computer before spending money on something, that may not work at all...