Scratch

Depends what you mean by 'cheap'. I heard that Rasberry Pi (around $30) can run Scratch relatively well, but if you want more speed, you should concentrate on something that has a high clock speed CPU, since Scratch currently does not support multiple cores or 64-bit (limit of 2GB RAM).

A cheap netbook would also run it ok, but they usually start at $250.

Scratch 2.0 will be programmed in Flash, so you would have more choice. Any sort of computer would work good with it, since Flash has 64-bit versions, and multiple cores support afaik.

You could get the Macbook Air base model. I've found that Macs run Scratch faster than PCs with higher specs.

Btw, graphics card doesn't really matter.