I can't remember if this is correct, but I thought for some reason the 64 and 6500 ran with the same board.
If so, you should be able to drop a two or 4-port in with little difficulty.
It's been awhile, though, and I can't remember, but I'm sure Kelly will chime in here soon.
The 5500 has the identical board that my 6500 has and my pastor, amidst my objection at the attempt, since I had read you couldn't do a combo or 4-port in that old of box, nevertheless, got it to work just fine.
He bought one off the shelf at the CompUSA (4-port USB) for something like $17.
You'll need to download the USB Card Adapter Support from Apple, then install and you're good to go.
Take care.
Chris