If you can afford it, by all means, go for the highest-clocked Tbird your budget allows you. If money is a concern, go for the fastest Duron you can afford. Duron is really on-par with Tbird in all practical gaming uses because video card is the limiting factor in most situations. Comparing Duron against a Tbird:
+ better price to performance ratio, no doubt.
+ even the slowest 600MHz Duron unoverclocked is fast enough for almost any normal use. Todays CPUs are overkill for gaming.
- ~10% slower than equally clocked Tbird.
- can't overclock as high. Not many Durons make it over 1GHz, but most 900MHz copper Tbirds seem to hit at least around 1.1GHz with sufficient cooling
Choice is yours. I'd pick up a Tbird in the 900MHz-1GHz range, and not overclock it, but that's just me 🙂