who did you no favors there.
OS X 10.5 began life in 2007 and had 8 updates. You have the latest version of 10.5
There was really no reason why you should not have updated to OS X, other than the minuscule cost.
Your MacBook is actually capable of running the latest version of OS X, 10.10, but I doubt you could make the leap from 10.5.8 to 10.10
That said, you could possibly move directly to 10.6, Lion, but it was not free. You will need to find a set of install DVD's. Perhaps eBay or Amazon. Around $30
Add all the RAM you can to that MacBook and you run 10.9 easily.
There's no "supposedly" about it, El Capitan, 10.11, will be released in the fall and if it is anything like the last update, Yosemite, it will be free.
Updates have always been free, as long as you were within the major release number, 10.5.0 through 10.5.8 for instance, but major releases, 10.5 to 10.6 were normally paid. Recently though, Apple has been making these free as well.
Go ahead and get the MBA now, I'm pretty sure that 10.11 will be another free upgrade.
P
I have a MacBook with a really old OS -- OS X version 10.5.8.
When there were updates available, an Apple-using friend advised me that I didn't need them. So now, I need to update some browsers and my laptop's OS is not up to snuff to do it.
I've had the laptop for a number of years, and by all accounts I'm ready for an upgrade (getting an Air). Supposedly Apple's El Capitan OS is coming out "in the fall." But because I need to work from home coming up in a month, I feel I need to get it now.
The question: If I get it now, will Apple update my laptop's OS for free when the new one comes out? Or are the updates all free anyway?

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic