This will depend what you want to test.
The Mac O/S X varieties really need late G3 processors and some real ram behind them as a minimum.
Have you tried surveys of the user population for the application to work out the best combinations?
There are many prematations and this will depend on what you are looking for with the testing, ie are you looking to certify to a specific configuration or all possibilities in the intended user base?
Best bang for your buck would be a quicksilver g4 unit that could be configured as dual boot(OS 9.22 and OSX).
Install the last 2 versions of I.E, Netscape and maybe AOL on the 9.22 partition.
Install safari(comes pre installed usually) and any other required browsers on the OSX partition.
Much of what you test depends upon the target demographic of the company hosting the site under test. Server logs, marketing info and the general business the company is in should help you determine how much you test.
For example- Large online sellers usually want extensive multi platform/multi browser validation... exclude no one from buying.
small companies using their site as a marketing and communications vehicle usally are satisfied with Windows browser testing.