Why don't you want the A600? If you set them both up with a null modem cable you'll have a fantastic multiplayer setup, there are heaps of games that make use of this kind of link.
Maybe in the spirit of Christmas, someone will find some spare hardware to give to you for your A500 as a gift! I hope so! I wish I had some spare 512k/RTC expansions, but I'm in need of a couple of those myself since the batteries exploded and killed them.
If you can afford to pay, you could email this guy and ask if he could make you one of his A500 expansions -
http://www.students.tut.fi/~leinone3/ide/ide68k.html
