Windows 7 is worth it, IMO. I've had nothing but great times with it since the beta even. I believe you can do an upgrade install from Vista 64 (as long as SP1 is installed) to Win7 64. I think you will have to get win7 ultimate - the OS has to match (home -> home, ultimate -> ultimate).
If you want to speed up Vista first before you try Win7, check out the Black Viper pages on tuning windows services.
http://www.blackviper.com/
I would recommend doing this for anyone, windows of any flavor has too many services that are just not needed for most. Ultimate includes the most bloat for the average user.
Now, if you get win7, even though you might be able to do an upgrade install, it would be much better for you to do a clean install. Yes, it can potentially work without a hitch, but it will probably cause problems in the future. Best thing to do is back up all the important stuff, and reformat the drive before installing. BTW, the upgrade version is meant to upgrade the existing windows - but that doesn't mean you have to install or have installed windows before. I think it asks for your existing key or something like that, but you don't need to have it installed.