Hi Lisa,
A computer purchased in January 2007 would have come with OS X 10.4 "Tiger" on it, as Leopard was not introduced until October of 2007.
If you buy a "retail" copy of Tiger, Leopard or Snow Leopard, you'll lose out on iLife '06. Leopard and Snow Leopard come in a "box set" bundle that include iLife and iWork so if you go the route of buying a retail version of the OS, I'd recommend the box set.
Also, AppleCare (if you call them) can sell you the replacement discs that would have originally come with your computer when you purchased it. This will give you the OS and iLife for a whole lot less than the cost of buying a retail box set.
Your computer would appear to be this model:
http://support.apple.com/kb/SP23 (MacBook, Late 2006)
Your computer most likely came with 10.4.8 preinstalled (released 09/29/06), at worst it was 10.4.7 (released 08/07/06). It would haven't come with 10.4.9 preinstalled (released 03/17/07).