Well, this is never an easy situation. Many manufacturers just don't get it, and they do not bother to release an OSX version of their drivers. (But then, they are also not moving towards supporting Vista either!) Having been through this process, it can be frustrating. I would suggest buying an adapter wherever they have a good return policy. Perhaps it will be a bit more expensive but you won't get stuck with duds. Manufacturers use different chipsets, even in runs of the same model, so the model number may or may not help you.
Any Broadcom chipset will work natively, and will be detected as an Airport card. These are a little hard to find. Fairly common is the RaLink chipset, which have made a good driver available for OSX. Both Belkin and DLink use this chipset in their newer adapters. We have had excellent luck with the Belkin, and the DLink WUA-13xx adapters they sell at The Source. The DLink G-122 ued to work, but the newer versions have a different, unsupported chipset that will not even be detected by the Mac.
Some NetGear adapters use compatible adapters. However, the inexpensive one (I can not remember the model number) that seems to be everywhere does not have a "chipset" per say, it requires the Windoze only driver for the data pump, and hence, will not work under any other OS. Buffalo used to make a good adapter, some with the Broadcom chipset, others with I think the Prism II chipset. Newer ones do not have drivers available for OSX, perhaps using the same "RIP chipset" used by NetGear.
ZyDas based cards work, but since ZyDas went out of business last year, it is obvious that they do not supply a Leopard driver. There may be aftermarket drivers that will work. As a rule of thumb, if it works under Linux, there is a good chance it will also work under OSX.
The next point is performance. Even though the adapter may run at "G" speeds (or perhaps even "N" speeds"), unless you have a Mac that has USB 2.0, you will only be able to obtain "B" speeds. However, since many Hotspots only allocate a "B" speed channel per user (because of multiple users needing access), this is less of a liability than one may think. If you are using it at home, well, you do have a number of good, if less portable, alternatives if you really need a little extra speed.
