Originally posted by: Puffnstuff
This is what the vista help file says on readyboost.
Windows ReadyBoost can use storage space on some removable media devices, such as USB flash drives, to speed up your computer. When you insert a device with this capability, the AutoPlay dialog will offer you the option to speed up your system using Windows ReadyBoost. If you select this option, you can then choose how much memory to use for this purpose. However, there are some situations where you may not be able to use all of the memory on your storage device to speed up your computer.
Some universal serial bus (USB) storage devices contain both slow and fast flash memory, and Windows can only use fast flash memory to speed up your computer. So if your device contains both slow and fast memory, keep in mind that you can only use the fast memory portion for this purpose.
The recommended amount of memory to use for ReadyBoost acceleration is one to three times the amount of random access memory (RAM) installed in your computer. For instance, if your computer has 512 megabytes (MB) of RAM and you plug in a 4 gigabyte (GB) USB flash drive, setting aside from 512MB to 1.5GB of that drive will offer the best performance boost.
The limitation is on the fast flash memory portion of a standard usb memory stick. If you get a device like the corsair turboflash which is vista certified you can use all of it. In the description above ms states drives which is plural meaning more than one. I haven't seen an actual limitation to this capacity so if you know where it is please share with the group. Thanks.
Edit: I found this at ms which talks about the 4gb ram limitation and how to get around it in an x86 machine.
MORE INFORMATION
A 32-bit operating system can address memory that is relocated above the 4 GB boundary if the following conditions are true:? The computer is in Physical Address Extension (PAE) mode.
? The computer has 4 GB of RAM.
In this case, the operating system correctly reports how much memory is installed.
Additionally, some x64-based operating systems can address up to 2 terabytes (TB) of RAM. For more information, visit the following Microsoft Web page:
http://www.microsoft.com/technet/windowsserver/sp2/overview.mspx (
http://www.microsoft.com/technet/windowsserver/sp2/overview.mspx)
To enable PAE mode, add the /PAE switch to the Boot.ini file. The Boot.ini file is in the root directory of the system drive.
You can read the whole article here:
http://support.microsoft.com/kb/929580/en-us