Most bang for the buck would probly be the 600, at 50 bucks you can't beat it, but, you're looking at the Duron 650 or T-Bird 750.
Probly best bang per dollar would be the Duron, for almost twice as much you get 100MHz and a bigger L2 cache. In tests a t-bird only outdoes an equally clocked duron by 5-10%. But, nothing beats the bragging rites on a thunderbird. If you've got the money, go with the t-bird.
Also, SETI@Home runs better on a t-bird due to the CLI fitting perfectly in the tbirds L2 cache.