If I'm not mistaken its 320GB of HD and 2GB of RAM. I have 2GB of RAM and a 7200 RPM 160GB HD in my mini and that bastard is a little screamer for what it is.
It can take two 1 gig PC5300 SO-DIMMs and it can take a 500 gig hard drive internally (the largest currently available), and a hell of a lot larger than that externally. I have two 1 TB drives connected to mine. A firewire media drive and a USB Time Machine drive.
If you want to hack it, you can attach an internal SATA cable to an eSATA port out the back, and connect it to a 1 TB 3.5" eSATA drive. Of course, you lose your internal drive doing this and you'll only get 1.5 gbps, since that's what the controller can do.
It just depends. With my 7200 Hitachi and my RAM my mini boots really fast. That said I never restart it. Its alway sleeps. Its just good to have when I am rendering photos in lightroom.
An internal drive will smoke any USB or firewire drive that you can connect to the Mini. Much faster read, write and random access times for most purposes.