I'm running a MB 3.1 late 2007 with 10.5.8.
I'm so surprised at all these people telling/selling Snow Leopard to those with older machines.
When Snow Leopard came out I considered upgrading my machine. However a lot discussion about hardware incompatibilities and 'the need for speed' which my MB 3.1 just dont got, despite a majeur boost in memory, had me hold out.
If you've an older Iphone that can run IOS6 my solution (scroll down a little) won't put your MB at an 'infarction'/total crash and death risk via an unadapted upgrade! Ask apple customer service about your machine and compatibility....(the day my 5 arrived, I called in a panic to see if it would be possible to upgrade and any issues/solutions they had and they clearly said...uh yeah it could be a problem.......after trying to immediately sell me Snow Leopard.
Otherwise a newer MBP is your best bet, (if your MB won't support upgrading) as 10.5.8 is going totally obselete and thus security issues on the up.