As I'm sure you're aware, Windows Vista introduced a feature called ReadyBoost which allows the operating system to make use of storage space on a USB drive or SD card to extend the available system memory. So if you have, for example, a 1GB USB drive handy you can plug it into your system and get a corresponding performance boost.
Now the problem with this is that all USB drives / SD cards aren't created equal and some may be too slow to be usable. If you do a quick search of the internet you'll find lists of compatible devices but I thought it worth mentioning something else you can try, mainly because I just tried it and managed to get over the "this device does not have the required performance characteristics" message. Simply try reformatting it using a different option (ie by right-clicking on the drive in Explorer and selecting "Format..."). Sometimes it seems to like you choosing NTFS as the file system, sometimes FAT32. And you can try playing around with the allocation unit size too.
For the 1GB drive I have just plugged into my machine FAT32 and 4096 bytes of allocation size did the trick. But it took a few attempts with different format settings to get it to work. So is my machine significantly faster? I''ll get back to you on that, at the moment I haven't noticed.
Alternatively, and if you're desperate, Windows Vista magazine has a slightly more radical approach that involves playing around with the registry. But be warned that a hack like this might not actually result in a faster system...