Q: Can't upgrade to mavericks

The hardware requirements for Mavericks are give in http://support.apple.com/kb/HT5842

Do you have enough memory and hard drive free space available?

That is different...you must have a MacBook5,1 or later to install Mavericks, the MacBook4,1 cannot use Mavericks.  The system is not a full 64-bit firmware system.

That is a difficult spec period as Apple made a couple different MacBook models designated the same time period but the hardware models are just different enough to not work...this comes up now and then with users and can be hard to sort out.  You should be able to move up to Lion if you want, that is a $20 download you get by calling Apple at 1-800-MY-APPLE if you are in the US...they send a download code and instructions in an email after a few days.  Lion will give access to iCloud if you want that access.