Your choice,
Personally I've never upgraded a version of Windows, you might find that you have to re-install half you software again anyway, and you never know what files may be left over from your old system that are just taking up space and aren't needed, etc.
Win7, I kept the old HDD / installation as a backup for my files and did a clean install to a new 1TB HDD (I bought months before but never got round to fitting it).
Also cos my WinXP was infected with a virus might have had something to do with my decision to move to Win7.
Others that I know:
Toesy, did do an upgrade which lasted 2-3 weeks, but then had a few issues with MW2 (I think, maybe something else as well) so he then did a format and clean install.
Scotty, clean installation. Had a few issues crashing on the installation , but then a bios upgrade fixed the problem. Then installed perfectly afterwards.
__________________