You need a ready boost enabled pen drive. As these have faster access speeds, they are a bit expensive but in terms of reliability/durability can't match RAM. So its better to upgrade RAM rather than using that solution.
AFAIK DDR2 RAM'S are dirt cheap and another thing is that the life of the pen drive will reduce drastically due to frequent read/write cycles. And to use it in vista you get some option in the right click context menu of the pen drive