Consider adding a second monitor exactly like the one you already have. Upgrading to a second monitor ihas been one of the most satisfying upgrades I have ever made.
A larger monitor would be nice, but it is bothersome to drag windows across two different sizes. You should be able to get a 22" for <$150.
You might be able to get a 24" for <$150 on craig's list.
I love the performance of SSD, and that would be a great upgrade.
But, I would hold up on a SSD for a while. This fall should see the launch of gen3 ssd's which will be bigger, faster, and cheaper.