Bang for buck is and always has been the 600, if you can get them. They do not make 600 anymore. I was fortunate to get a tray of 600's today and so far many are running at 1GHz. It's nice to be able to run them at 133MHz fsb on the new chipsets. 7.5 x 133 is a good little performer.
I've had plenty of 650, 700 and even 800 that would not do 1GHz, but, yet cost more. Go figure. :Q