I really doubt there is a issue with plugging headphones that are 50-Ohms or higher, into the GSX 1000.
Sennheiser recomends headphones in the 16-Ohm to 150-Ohm range, I would assume 250-Ohm beyers would work fine.
If you went with a external DAC/amp, you would want a DAC that comes with an optical input, as optical will allow you to use the features of a sound card.
The optical setup can be used with on-board audio or a cheap sound card (used Sound blaster Z?)
Where as a USB DAC will bypass the features of a sound card.
Creative Labs sells headphone surround sound software, for $30, that works with most on-board audio processors.
Maybe just buy a Creative Labs AE-5 (used eBay), everything you need in one unit.
If i end up buying a sound card, ill probably go for one of the new sound blaster's. That's what i'm trying to figure out though. As far as i can tell, the only difference between a dac/amp setup, and a soundcard, is the software. The question then is, is this software a "godsend" for what my goal is here, or is regular stereo playback with an amp to drive the headphones correctly better. These are the questions i need input on, based on experience preferrably.