I actually used to have one of these computers and surprisingly I got about 45fps on medium with Optifine. Fantastic processor, I actually used to run a GameCube emulator on it. It is upgradeable though not by much, 8GB ram and a low pro graphics card is about as much as you can do to it, could put an SSD in it and thats about it. All in all a very sanzzy computer.

Normally, without optifine really bad, like 20fps lowest settings on superflat
Butwith optifine settings turned all the way down, easly 120 fps on superlat

I have a potato laptop and get 11fps on superflat without optifine
But i get 110 on superflat with optfine

My Specs:
4GB RAM
Intel Celeron Processor (1.10 Ghz)
Intel Standard Graphics
500 GB HardDrive

You can easily get over 100 on lowest settings and around 60 on highest settings

Perhaps 20 FPS on lowest settings. It'd be about 5 FPS on high settings. This isn't for sure, but I think that's about right.

with the lowest settings, 0fps as a potato cannot run anything