How to speed up old MacBook Pro15 inch late 2007 with 4GB RAM (2.4 GHz)
Things to consider - you'd have to check whether any of these are worth the money on such an old machine though.


Larger, faster HD, or even an SSD. You don't say how big the existing drive is but those machines came with 120, 160 or 200. In any case, 12GB spare is really not enough for the machine to 'breathe' properly.
Assuming it's never been done in the life to the machine - defrag, using iDefrag (only good to Mavericks, not Yosemite) or similar - bear in mind this may take more than 24h to complete, especially with so little free space.
Add more RAM - that machine will take 6GB, as a 2GB stick &amp; a 4GB stick. I would always use something like Crucial's configurator app to be sure you're getting the right thing.


Data Source: EveryMac