Fomalhaut30: Well, a fair bit to think about. Something else...is there a connector adapter for the Sidewinder I do have? My current comp doesn't have a port for it, ttbomk. If I could get an adapter, I wouldn't necessarily need to get a new joystick.
There are db15 (gameport) to usb adapter. At least once upon a time, there were such devices. The problem is, there were 2 or 3 technologies. one was a mere Db15 to Usb but would only work with a few joysticks that had the usb circuitry included ( sidewinder precision pro I think ), others had a converter but would only provide an analog mode
a true usb joystick will probably be a safer bet , espacially as those adapters are unlikely to work with XP, let alone Vista and 7...
Another solution ( for a desktop ) might be one of the few cheap 4 channels sound cards (PCI) on the market. Chinese built, poor sound but... gameport. But still...
edit 2 : for soundcards with gameport, check whether the gameport works with newer OS. It might be limited to XP 32 bits ...