You need 64-bit to really use more than 3 GB of memory. In 32-bit Windows, the addresses above 3 GB are reserved for hardware use. So if you have 64-bit XP, go for it. But Vista Ultimate comes with 64-bit and 32-bit version, so you can easily reinstall if you want to try things out. Here's a good article on the 3 GB limit http://www.dansdata.com/askdan00015.htm