Why not wait just over three weeks and upgrade to Windows7 (that's if you're really determined to upgrade, which IMHO is not justified from a perfectly capable OS).
By all accounts Win7 is significantly better than Vista - faster and lighter on resources.
If you do a clean install you'll have to reload absolutely everything - with a clean install nothing at all is retained (that's why it's a clean install).
You can't use any MS OS on two different systems at the same time - thus continuing to use XP on one system then using it on another system, even if you upgrade it to Vista, is strictly speaking against the licence agreement you have accepted. If you are going to use the upgraded cloned drive on the same system - that would be OK.