I'll give you some information:
Upgrades are always worse than fresh Installs:
1) You still retain half the [censored] that was on previous OS(apps garbage, virii etc that you may or may not want).
2) Registry information is also ported over, take that for what you will.
3) Doing a clean install will allow you to only install the applications and documents you NEED, no extra clutter. That comes later when you use the computer for many months =).
Windows 7, is FAR FAR better than Windows Vista. Everything seems far more responsive, less bloat, peppier and the UI just feels natural. While Vista was a dressed up XP, Windows 7 is a refined Vista with a Linux style/Mac OSX task bar. Its a nice touch and adds functionality. Memory management seems far better in 7 too, as if they really tweaked it and improved it. Vista just seemed to run smooth, then would chug. My experience with both the 32-bit and 64-bit versions lends me to say even if you have only 2GB of memory and a good 64bit CPU, install the 64bit Windows 7.
32-bit Vista ran like garbage on my Fiance's laptop:
1.6ghz C2D
2GB Memory
Intel GMA 950.
120GB 5400 RPM HDD.
Same laptop, Windows 7 just flies, Firefox, IE8 etc all open quickly, there is no "waiting", things just open, software works. Driver's are already good to go(you can use the Vista drivers).
My laptop:
2.4ghz C2D Penryn
4GB Memory
Nvidia 8600M GT 256
320GB 7200 RPM WD Scorpio
64bit Vista to 64bit Windows 7, night and day difference in terms of speed. Boot up time is faster, OS litterally "blazing" fast.
Fiance's laptop is able to play games that she normally was not able to, [censored] SIMS 3 runs SMOOTH under 7, and barely playable under Vista. From a usability and gaming perspective, Windows 7 is XP Enhanced and Vista -reworked. Meshing the two, and creating something that works. It's everything that should have been Vista but better.
I suggest you give the Windows 7 RC1 a try, its free for 1 year. I used XP up until the Vista Alpha was released years ago, used the Alpha, Beta, RC1 of Vista, bought Vista and used it through 32-bit to 64-bit and Windows 7 trounces it. I'd invest in an external or secondary hdd and back up your important documents, games etc onto the drive and give 7RC1 a try.