Not sure how this whole Windows 7 upgrade thing will work for you? Well, this official Microsoft chart probably won't help one damn bit.
Many of the upgrade paths here are illogical and unlikely to come into play (Vista Starter 64 to anything for example) but to simplify things a bit, if your upgrade path falls in a green box, congratulations—your upgrade process will be easier and will preserve your current files, settings and programs. If you fall in the sea of blue there, that means you will have to perform a "Custom Install" which falls somewhere in between an upgrade and a fresh install. If you are still confused, head on over to ZDNet and check out a simplifed version of the chart. [Mossblog via ZDNet]