It's more a function of your on board RAM. MS say that it can help up to 150% of your on board RAM. So if you have 1GB of RAM you can benefit from up to 1.5GB of Readyboost.
But not all cards work because for Readyboost it has to meet performance criteria which relate to read/write speeds. Many cards are optimised for fast single event speeds rather than constant speed of access.
The best approach until new readyboost designed cards arrive is to try one you already have. You may have spotted that Vista has a tool which will test the card for suitability.
Finally, tests have shown that RB makes little or no difference to machines with good RAM (say 2gigs) but there is an improvement for those with 512-1Gig.
Remember its not the same as RAM so the benefit will not be the same as adding RAM direct.