I don't know about the Nv15 demo. If a cat sh*ts on the floor, I always make someone else clean it up. Same deal here.
As far as Quake3, it's a tie. 50% of the time I clean it up. The time demo is just a standard 'movie' that runz using the quake3 demo, and comez back with a max and min fps. So, it's like a benchmark that is standard, no matter what computer you are using. Pentium4's always 0wN on this one.
UtBench is the same deal. It's another cut-scene using the UT engine. It won't give you a max/min though, so you have to watch it really closely or have it log to file and search through the damn thing afterward. It's much more processor intensive, so you see the FPU work a lot harder to hammer it out.
Dunno about Undying. If there is a console command 'show fps' or something, then yeah; you can bench with it.