I can't give you a personal opinion on the matter because I quite frankly don't own a Media Center PC. However, the information that I have read indicates that you should check with your PC's vendor to ensure that an upgrade to 2005 is even possible.
You might find these resources helpful:
http://www.microsoft.com/windowsxp/m...r/default.mspx
Microsoft's official page
http://en.wikipedia.org/wiki/Windows...Center_Edition
Wikipedia page on MCE
http://reviews.cnet.com/4520-3672_7-5536454.html
C-Net review of MCE