Hello,
What you have there is a G4 Tower computer that ran at 400 Mhz, with some expansion options. It is not the newest / fastest beast on the market, but for a good used price, it will make someone a nice home computer, or in the office for moderate tasks.
What do you plan on doing with the machine? If you need the fastest, newest, bang-for-buck, this model will not do it for you. Latest Macs are dual processor, 2 GHz units... much faster than this one. If you are doing digitial video / photo imaging, or running Quark / Indesign / Ragtime, you will probably want to look for something faster / newer.
If you are looking for college student type things, home use, non-time-critical things, it will be a nice computer, and serve you for a few years. It is faster than mine at home, and it looks like a nice machine.
I would deck it out with 512 MB of RAM, and put a CD/DVD burner in it though.
Christian