Most people go to Windows 7 when they buy a new pc. I have trouble putting good money into a old computer. But it does make some old hardware feel new again. Henydiah lists the min speck I would say that you need at least 2gb of RAM to run the 32 bit OS but I would recommend 4GB of RAM and the 64 bit OS.
Hi maxwellhouse1 - With Windows 7 you get access to a lot of cool features that you cannot access with XP. Also, many companies have stopped creating new software for XP as the Operating System is over 10 years old. I'd say it's worth upgrading simply to stay current.
With that said, I also agree with anonymous1. If you're running XP on an older machine, it may be worth it to purchase a new computer with Windows 7 pre-installed. That's only a decision you can make