Windows 7 supports Windows Ready Boost. which uses external USB flash drives as a hard disk cache this improves disk read performance. Thumb drives, SD cards work good but will not support this if the primary disk is an "SSD" Solid State Drive, there will be no significant benefits.
The best way to test to see if your flash drive will work as Ready boost is to test it! Windows Vista and Windows 7, when you insert the removable media right click on the media when it shows up, and select use as Ready Boost, the system will tell you if it is fast enough to run as ready boost. Usually if it is a fast memory card like one for a digital camera, or such that works best, like a "class 4" or higher should be just fine to notice a significant speed gain!
And Remember now a lot of packages are coming with the product saying "Windows Ready Boost" compatible!