Your config looks fine to me (at the first glance)...I'm not actually sure about this, but there is one thing regarding 3d-cards: usually (so I've heard) 3d-hardware stuff won't run on resolutions over 1024x768 too well. So that _might_ be the reason...altough, as I said, I'm not sure about this (meaning: shouldn't X still run on higher resolutions, but without 3d-hw?)
Are you sure your card/monitor does support that video mode?
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