They should perform well. I am using basically worse of what you're using, and I can run Minecraft decently.
It's one of those games where you need a good bit of RAM and good CPU, which I think 4 GB is fine, and I think your CPU is fine too. The graphics card is also good, at least better then mine. I have experience with Portal 2 and Minecraft, but not Dirt I am afraid.
I can't give you an exact FPS you'll get, but I am sure it'll be a good amount for Portal 2 and Minecraft, it all depends on what mods you want to use for Minecraft rather or not it'll work fine.
Hope this helped some.