I've also been doing a little research on the same topic and this is what I came up with.
This is a quote from windows.microsoft.com
Here are some tips on what to look for when selecting a USB flash drive or flash memory card to use with ReadyBoost:
The minimum amount of available space recommended for ReadyBoost to effectively speed up your computer is 1 GB.
For best results, use a flash drive or flash memory card with available space of at least double the amount of memory (RAM) in your computer, and preferably four times as much memory. For example, if your computer has 1 GB of RAM and you plug in a 4 GB USB flash drive, set aside at least 2 GB on the flash drive to get the best performance gain from ReadyBoost, and preferably the entire 4 GB. How much memory you need depends on how you use your computer. Keeping a lot of programs open at once uses more memory.
Give ReadyBoost 2 GB to 4 GB of space for best results on most computers. You can reserve more than 4 GB of space for ReadyBoost on most flash drives and flash memory cards. (Storage devices formatted with the older FAT32 file system can't store more than 4 GB.) You can use a maximum of 32 GB of available space on any single removable storage device with ReadyBoost and up to 256 GB total per computer (by inserting up to eight USB flash drives or flash memory cards into the same computer).
To work with ReadyBoost, a USB flash drive must support USB 2.0 or higher. Your computer must have at least one free USB 2.0 port where you can plug in the flash drive. ReadyBoost works best if you plug the flash drive into a USB port directly on the computer, rather than into an external USB hub shared with other USB devices.
If you want to be sure a USB flash drive works with ReadyBoost, look for a note from the manufacturer that the flash drive is "Enhanced for ReadyBoost." Not all manufacturers list this on their packaging. If there is no mention of ReadyBoost compatibility, the flash drive still might work with ReadyBoost.
There are many different kinds of flash memory cards, such as CompactFlash and Secure Digital (SD) memory cards. Most memory cards work with ReadyBoost. Some SD memory cards don't work well with ReadyBoost due to issues with the SD card interface. ReadyBoost will display a warning message if you attempt to use one of these cards.
Now, on a personal note, I tried ready boost for myself on my laptop which runs Windows 7 (64-bit) with six gigs of RAM and an i5 processor.
I used a 32 gig SanDisk cruiser flash drive reformatted to ex–fat format so as to dedicate the entire flash drive to readyboost and I did notice that files open a bit faster than before.
I tried opening some pretty large files and I can tell you that I definitely noticed that it takes less time than before.
So, my conclusion is that although my laptop has six gigs of RAM, dedicating that 32 gig flash drive to ready boost definitely was worth it. The thing is, I really don't want a flash drive sticking out of my laptop so I just ordered a 32 gig SanDisk SD card to see how that does because my plan is to leave it in permanently, I will keep you informed as to how that goes.
P.S. FYI, not every flash drive or SD card will work for readyboost, I tried a SanDisk Extreme SDHC UHS1 SD card with a  rating, suitable for advanced cameras and HD camcorders and it did not work with readyboost. I have now ordered a 32 gig SanDisk Ultra SD card which I believe would work because I've tried a two gig version of the Ultra that has a  rating and it worked great.
I will let you know if I was able to use the entire 32 gig SanDisk ultra for readyboost.
Remember, You can use a maximum of 32 GB of available space on any single removable storage device with ReadyBoost and up to 256 GB total per computer (by inserting up to eight USB flash drives or flash memory cards into the same computer).