Don't use a spreadsheet. I tried this my first year in business and it was HORRIBLE. It is very difficult to back up from a tax standpoint. I switched to QuickBooks a couple of years ago and had a bookkeeper help me set it up right the first time. It tracks everything and does a wonderful job at tax time as well as just letting me know how I am doing through the year.
Quickbooks would be the way to go. I think the Simple Start edition would work for what you are doing, and I was just on their site (quickbooks.com) and see they now have a free version of the Simple Start edition for download. Also, they have an online version as well (which has a 30-day free trial).
Setting up a company for a consultant or independent professional is fairly easy, and the help and documentation can provide guidelines on what to do. Also, when you get an accountant, Quickbooks exports and reporting is something most are familiar with.
I'm using qb 2006 pro. My accountant has that version among other versions. I was using and older version and he had to spend some (billable) time each year getting things into the format he was working with.