If you have the hardware and software to do so, I would seriously consider cloning the hard drive on your current system, so that you can go back to XP for a short time in case you run into any snags.
I had one upgrade license knocking around from when I bought a three-pack a couple years ago. In my experience, with that kind of license, there were some snags in the activation process, so I had to go back and boot from the cloned XP drive and do it all over again following slightly different rules during the installation. (This wasn't too hard for me, since my case has a hot swap drive bay in front.)
That reminds me of one other thing: my understanding is that XP doesn't support Advanced Host Controller Interface (AHCI). From what I understand, it's faster and more reliable to run that way, so you should find the BIOS setting and change it before you upgrade.
One final thought: if your system is pretty old, you might be able to replace it for not much more than the price of a Win7 license. Something like this 4 to 5 year old HP system for $100 with Win7 home premium might be just the ticket. http://www.newegg.com/Product/Product.aspx?Item=N82E16883280004
I bought something like this to replace two older XP systems at church. (Then you could experiment with Linux on your old system.)
I'm new here, so I trust that if I've said anything stupid, someone smarter than me will chime in.