Dont expect amazing results..
Logitech QC3000 pro is what I have, pretty good quality, it has a motion detector feature so when u leave it on, it snaps shots
Dual P3 800 w/512MB Ram, on 320x240 it took from idle 5-8% CPU to 35% CPU and on 640x480, it took about 55-65% CPU
There isn't a lot of USB cams that has it's own onboard processor to reduce the load.. the other one was 600 bucks but didn't have good w2k support. Check the usbworkshop.com, you can read the horrors of working under w2k...