Facts:
Duron L1 cache = 128kb
Duron L2 cache = 64kb
Tbird L1 cahce = 128kb
Tbrid L2 cache = 256kb
So basically the Duron is a Tbird - 192kb cache.
Does it make that much of a difference?
Depends.
With my system in my sig, I pull off almost 4800 3dmarks, running the default bench.
Don't forget 3dmark2k is optimized for geforces.
In CS, I pull off a constant 100fps @ 1024/32bit.
I crunch seti @ home WU's in about 5.5 hours, and pull off over 3.33mkeys/sec in RC5.
For a chip that costs $50....that's not shabby at all.
Duron = Budget performer
Tbird = All out performer
Take your pick.
My recommendation: Look at the week of the processor, get the latest possible (IE: Pick a week 40 over a week 30)
It'll overclock better.