This has been tested by others before, mostly with Vista though. The general conclusions have been that it doesn't offer any significant amount of system performance improvement.
My understanding is that ReadyBoost is somewhat similar to a paging system, only instead of using slower traditional HDDs, it's designed for somewhat faster removable media storage devices, and/or perhaps as a kind of alternative to SuperFetch. (ReadyBoost - Windows 7 features - Microsoft Windows
"ReadyBoost is designed to help when your PC's memory is running low. Low memory can make your computer sluggish because Windows, which needs a place to stash data, turns to the hard drive. Flash memory offers a speedier alternative.
") The main advantage comes only from using ReadyBoost with systems that don't have sufficient RAM. If you have more than enough RAM, considering you have enough extra to make a RAM drive, then you likely won't get much if any system performance improvement by using ReadyBoost, even though you are using it on the much faster RAM drive.
You might try using the RAM drive for temp files, browser cache, paging space on 32 bit systems that can't otherwise use the extra RAM, and for temporarily storing files and running programs that are frequently read from and written to (such as games that often load new areas, or video/audio/photo editing programs and files.)
Also, keep in mind that Windows 7 does a better job of proactively using available RAM with features such as SuperFetch.