I think Tanuj was thinking of ReadyBoost, a feature in Windows Vista that lets you use a USB stick as an extension to your paging cache.
This is something that your hard disk is used for when you run out of RAM to do the work in hand. Caching this data on a memory stick, as well as writing it to hard disk, might speed up access to it when it's needed again. But this isn't adding USB memory to the pool of RAM, because, as Christian says, the bus speeds on USB are far too low for it to be useful as RAM, and that's not going to change in the foreseeable future. All this really does is give you somewhat faster access to things that your system has already overflowed from RAM onto disk.
More info on how it works, and its limitations, is here.
This is something that your hard disk is used for when you run out of RAM to do the work in hand. Caching this data on a memory stick, as well as writing it to hard disk, might speed up access to it when it's needed again. But this isn't adding USB memory to the pool of RAM, because, as Christian says, the bus speeds on USB are far too low for it to be useful as RAM, and that's not going to change in the foreseeable future. All this really does is give you somewhat faster access to things that your system has already overflowed from RAM onto disk.
More info on how it works, and its limitations, is here.