MINIMUM: 500 mhz processor, 96mb ram, 16mb video card, Windows XP, Mouse, Keyboard, Internet Connection
RECOMMENDED: 800 mhz processor, 128mb ram, 32mb+ video card, Windows XP, Mouse, Keyboard, Internet Connection
I didn't even have min specs and it rocked out. I think I had 400Mhz, 32mb RAM and an 8gb card? It won't run on a potato like Doom, but it was a shocking step back from the ever-increasing specs required at the time.
500mhz must be updated for what ever they've done to it- I remember playing it in 1999 on a 133mhz pentium on Windows 98 (and I also remember testing in on NT4 and it actually worked!)
It was running on a pretty old engine. It was based on the Quake 1 engine, but some features of the Quake 2 engine got tacked on along the way. There was some more eye candy added as well, but something with that basis isn't going to be as power hungry as Unreal.