Old pc case that I wanted to use. What kind is it? Obsolete yet?

I work in a lab and have found an old PC running windows 2000 on it. I want to take out all of the old junk and put in new hardware. Only question is, I don't know how much of the stuff will fit in. The system is an old Dell Optiplex GX400. Anyone know?

My main concern at the moment is the power supply fitting. The current one is 5.9" by 3.3" by 5.5", and as this is my first system, I don't know how big (or small) they get nor how they connect to the case. That's where what kind of case this is, and if powersupply-to-case connections are standardized or not (probably are), could help me out.

Anyone know?

Thanks ahead of time.