I recently posted that I would recommend staying with Windows XP as long as possible. Well if you buy a PC now, it is likely that you will get a coupon for a free upgrade to Vista when it ships. I would be very leery of what "free" really means.
As you may also recall from my prior posting, I suggested you should wait as long as possible so that Microsoft can correct all the security issues -- there will be security issues. PC World published on October 24th a good reminder about the new Vista Home Edition -- it will have less features than XP Home. So, for your free version, you can upgrade to an OS with fewer features. Why would you want to do that?
PC World makes another good point... upgrading your OS may not be an easy task either. Unless you are very comfortable troubleshooting your PC issues, I would recommend you avoid that chore -- wait until you buy your next PC in 2009. (Average users should be fine getting a new PC every 3 years.)