What's the best OSX for my 2007 3.1 MacBook 2.2Ghz Intel Core 2 Dual, 1GB RAM, currently using OS X 10.5.8? My ol' clunker is doing very well considering its age, but I would like to upgrade its OSX to see if it runs a bit faster, and also because I can't run the latest versions of some applications on OS X 10.5.8..

I'm just hesitant because I'm afraid to make the computer even slower with a newer version of OSX, even if Apple says that I can run newer OSX... Does anyone have any advice? Much appreciated!
According to EveryMac the latest OS that your Macbook can run is 10.7.5 (Lion).

A big focus on the preceding release 10.6 (Snow Leopard) was performance. It was the OS update that Apple advertised as having "no new features" as the work was focused on refining and improved the existing ones. Reducing the memory footprint, removing legacy code, the Finder was rewritten in Cocoa, a new Cocoa version of Quicktime was created, etc. In John Siracusa's review of Snow Leopard for Ars Technica he said "Should you buy Mac OS X Snow Leopard? If you're already running Leopard, then the answer is a resounding 'yes.'"

So I'd definitely recommend upgrading to Snow Leopard at least.