OS X actually slimmed down with Snow Leopard as the "Rosetta" support for PowerPC Macs was removed!
You may not even be able to upgrade - I have a dual-core Black MacBook of about the same vintage that seemingly won't go higher than Snow Leopard...
Yes, there are incompatibilities with some older old apps (Apple don't make a fetish of backwards compatibility) but if you can, do - there are lots of speedups and enhancements (and security tweaks).
Interesting that Mavericks was, (for the first time) offered as a free upgrade.
Mac
