Please do not upgrade an XP computer to Win7!!!! Windows XP is the most amazing software for its time. It can run on only 256mb.
Windows 7 needs min 1GB (I would never go lower than 2GB). So most likely your computer can not support it anyways.
That being said windows 7 the way to go, but you will have to buy new computers. Very nice ones from dell outlet - i3, 3GB, 7200RPM hard drive for $400.
