Replacing hard disk with SSD or increasing RAM in MacBook Pro
It depends on how much RAM you're using. Use something like iStat Menus or MenuMeters to keep an eye on your RAM usage.

If you're constantly near or over 4 GB RAM used (wired+active), more RAM will be the better option. (You can also tell this by seeing how much swap space you use; if it's over some nominal value like 64-128 MB get more RAM.)

If, on the other hand, you never even get close to 4 GB of RAM usage, get the SSD. Or, get more RAM anyway and use the excess as a ramdisk, which will be way, way faster than an SSD.