Bloom, it all depends on how often you need to upgrade.
I upgraded my current system two and a half years ago. I originally wanted to go with the cheaper socket370 Abit BM6, instead of the slot1 BX6 rev.2. Because of admonishments by others to think of the sacred upgradability, I changed my mind, and bought the BX6 rev.2, plus a slotket adapter, and a Celeron 466.
You know what happened. Slot1 got retired, and I could have paid cheaper to have a socket370 board. And now, I can't even buy a new processor for my BX6 rev. 2 (was max 700MHz). And I'm not ready yet to upgrade. Maybe 6 months from now.
That $40, I could have put on a better vid card (bought a TNT2 M64 while I though I was buying a "true" TNT2). I'm not gonna get suckered again.
It's all a matter of perspective, budget, and how often you really need to upgrade.
By the time I would be ready to upgrade again, I bet DDR memory will be obsolete.
BTW some KT133A boards can accept the Athlon XP too. It's just that they won't perform as good, but in general applications tests have shown only a 7-10% difference or less. Only memory-hungry apps show a (quite) significant difference.
Personally, right now, I would go with keeping my memory, buying a KT133A board with Duron, but buying a better vid card.
But... it's all a matter of perspective, needs and budget.