First off the MSFT site says that you should do a clean install, which I agree with, the RC will not be available after the 20th, so hurry, I got the X86 and X64 versions yesterday, and they are good till 1 June(or was that May) 2010, it's supposed to let you know 2 weeks before it runs out
Please do not build a new computer and stick a really old OS on it. Take full advantage of that shiny new hardware with Windows 7. Also, use the 64-bit version, as it will easily handle 4GB+ of RAM... whereas the 32-bit version won't be able to make full use out of that 4GB.
I recommend buying a copy of Vista that comes with an upgrade coupon for Windows 7. Vista is not as bad as you think it is... if you dislike the "theme", you can switch to Windows Classic to make it more like XP. Most likely, your laptop has a lot of other bloatware on it that makes Vista look worse than it really is. It's up to you what exactly you want to do, but I definately wouldn't recommend XP.