Dungeons And Dragons Online: DDO News: Minimum Requirement Goals

While it's still too early to discuss the actual minimum requirements, Xundau talks briefly about what they're aiming for, and confirms the initial posters system stats should be fine:

The Turbine engine that DDO is built on is very scalable. With all options maxed, it can give today's high-end machines and graphics cards a pretty good work out, but we also want the game to be playable by people who don't have the latest and greatest hardware. In fact, our goal is for the game to run on a machine with a GeForce 2 graphics card -- whether or not this will actually happen remains to be seen, but that's our target for the low-end of things.

That post can be found here.