MacBook 4.1 running 10.5.8 upgrade to Mavericks?

You need to first upgrade to OS X 10.6 Snow Leopard here.

http://store.apple.com/us/product/MC573Z/A/mac-os-x-106-snow-leopard

Then, use OS X Software Update feature to update to OS X 10.6.8. That'll give you access to the Mac App Store where you can decide if you want to upgrade to the free OS X 10.9 Mavericks, or not.

Before doing any type of major system upgrade or update, you make a backup of all your important data either by manual backup, Time machine backups or making a data backup using a data cloning app like CarbonCopyCloner or Super Duper using DVDs, USB flash drives or externally connected hard drive.

Also, your MacBook is seriously lacking the needed RAM for either OS X 10.6 Snow Leopard or OS X 10.9 Mavericks.

These OS X versions really need a working minimum of 4 GBs or greater.

Your MacBook can take a total of 6 GBs of RAM.

I, strongly, advise you install the max. 6 GBs of RAM.

Correct and reliable Mac RAM can be purchased from online Mac RAM sources Crucial memory or OWC (macsales.com).

OWC ( macsales.com) is the only online Mac RAM seller that sells that sells the 6 GB RAM kit.

Good Luck!

OWC (macsales.com) has videos for installing RAM into all year and models of Macs.

http://eshop.macsales.com/installvideos/macbook_13/