If you can afford to buy an additional hard disk (same size or larger than existing) rather than a new PC, then you have the option of cloning the existing disk to the new, then putting the new one in the PC / laptop and doing an upgrade, safe in the knowledge that if it turns out to be a mistake you can revert to your existing disk by simply putting it back. Alternatively, with your existing disk safely tucked away, you can also try a clean install on the new disk, and keep the existing as a backup - put it in an external USB caddy to make it really easy to transfer data (an external caddy also simplifies cloning).
Get your backup program to make as many images as you want. I use Acronis True Image. Simply do a complete disk backup to your backup drive (ends up being a large single file), then do whatever you want to do to the system drive. Works really well; I made various images as I progressively installed Win 7 and can roll back to any point along the way if I have a catastrophe or mess something up or just want to go back a year or so to get rid of the crud I tried but didn't like.
I'm going to try the upgrade route first. I'm getting too old to try new installs straight up!
If it doesn't work "out of the box", I'll wipe and start afresh.