I agree with the last post about the speed of USB in place of RAM - if you need something like readyboost on XP, maybe you should think of a memory upgrade.
I've heard of a few companies trying to come up with solutions to readyboost in XP. They do exist - just Google. Here's one:http://www.eboostr.com/
Never tried it and I don't endorse it, but I reckon it might be cheaper to sneak on eBay and buy more RAM instead. A few friends of mine say that XP already has the capacity to use readyboost by altering the swap file and put it on USB but that doesn't make any sense, really, unless you want to sacrifice RAM speed for RAM size. I sold my old 1GB RAM (2 x 512) in a local newspaper for more than I bought 2 x 1GB RAM on eBay.
From what I understand of Vista's readyboost, it's for older machines that can't accommodate the RAM the OS needs to operate.