I recently did this (a couple of weeks back), without knowing exactly what model it is it's hard to say exactly what specs you can "go up to".

I replaced my internal drive with a 128GB Crucial M4 drive, OS is installed here.

I replaced my optical drive with a housing and installed a 1tb 2.5" drive and located my home folders here. I would normally run filevault, but I'm unsure of compatability with the "unlock" tool to allow me to have the secondary drive decrypted at boot.

I upped my ram to 8GB as well.

Upgraded to mountain lion and I have to say the machine does fly!

A good place to check various compatabilities with stuff is OWC (http://www.macsales.com).

I've heard nothing but good things about the Crucial M4 and it seems to be the generic go-to drive for SSD updates unless you have specific reason for a different beast.

Adrian