Vista to Win7 is cake and you shouldn’t lose a thing (unless you have Ultimate in which case Windows Ultimate Extras vanish after a warning. Man I miss hold-em.)
XP to 7 is a bit harder, but most running XP could use a good refresh anyway. :)
I’ve done both upgrades and they went without a hitch.
You’ll like 7.
You will need at least 2GB Memory to make the switch to Windows 7. 3 GB Memory and a 1gb video card are even better.
Otherwise just go get a new computer when you have the opportunity.