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Now there are two models of Duron & two models of Athlon (actually its 3, the old Athlon Classic wich was using the slotA interface). The Duron (spitfire, Athlon T-Bird core w/ 64KB cache) came upto 950MHz. The Duron Morgan (Palomino core, 64KB cache). upto 1200MHz so far.
The Athlon T-Bird w/ 256KB cache, and the Athlon XP (Palomino core) w/ 256KB. the Only difference betwin T-Bird & Duron spitfire is the cache and same difference betwin Morgan & AthlonXP.
The Athlons have more cache so they are faster and better with multi-tasking.