Hi,
You can definately go for upgrade and can use Sandisk extreme 120GB the only thing that you need keep in your mind that you should be using OS X 10.6.8 atleast because by default TRIM feature is enabled from this version (TRIM command allows an operating system to inform a SSD which blocks of data are no longer considered in use and can be wiped internally)
it boosts the computer performance.
For TRIM you can go through the following link :
