Yes, the latest Durons should support 3DNow!, MMX, SSE, and SSE2. I'm not extremely positive about SSE2, but the other three are definite.
The only real difference between the Duron and a similarly clocked Athlon is the amount of L1/L2 cache.
~cdogg
[tab]"All paid jobs absorb and degrade the mind"
[tab][tab]- Aristotle