With XP you really need 3GB of memory to do 4-5 copies. The good news is you can get 2GB of Pc6400 memory for $12.99 at Newegg now (after mail in rebate, free shipping, search for OCZ PC6400).

As far as symlinking the data/cache folders, search the forums for a program called winbolic link - there should be instructions on what to do as well as a download location. This is for XP. For Vista you can do it inside the regular OS functions but I'm not sure how as i've never used (and have no desire to use) Vista.