Hi,
I am a hobbyist too and I am looking around which 3d program is payable for me. Have a look to http://home.tiscali.be/heyse/help/index.htm. This belgium guy have done a list of free/shareware 3d software.
I'll like to buy Carrara and I purchased at ebay Ray Dream, so I can upgrade. In the States it will cost $149, for me in Germany it's about 230 Euro. In ebay look for old programs, perhaps it won't work on your system. But if you have the serial number, you can upgrade.
By now in german ebay there you will find Ray Dream 4 or 5 for 15-20 Euro. It's equal for an upgrade if you get a MAC or Win version.
Take care

Wolfgang