How would you say those compare to something like Sennheiser HD280? Do you think the audio card is necessary? What I mean is, I keep seeing mixed opinions on whether a dedicated sound card is really necessary. Some say the integrated is good enough for basically anything short of professional, others - particularly in the gaming section of this forum - suggest the Asus. I would probably be getting the DGX (PCI-e x1), as that would future-proof it a bit, since PCI slots are well on their way out - is there any reason to pick one over the other besides preferred interface?
I have several nice headphone that are in the $250-$300+ range and the Gemini HSR-1000 is not that far behind compared to them.
Your budget falls short of the headphones ($200 or more) that makes a noticeable difference, you might get lucky with something used off eBay, with in your budget.
I've never listened to the Sennheiser HD280 Pro, but as they sell for under $100, Market forces have shown what the public thinks they are worth. even with a good name like Sennheiser.
(To me the Sennheiser HD558s at $130 are selling for less then they are really worth).
Integrated motherboard audio has been getting better every year, but I'm assuming your motherboard design is several years old and the company that made it's (AD2000) audio processor dropped out of that part of the market about 3 years ago, so I'm assuming there is not much in the way of driver updates.
Where as the Xonar DGX has drivers that have been updated 7 months ago (Unified Xonar Drivers).
The Xonar DGX has a basic headphone amplifier, where I'm assuming your motherboard uses a Line-out jack that pretends to also be a headphone output.
The Xonar DGX comes with Dolby Headphone surround sound, I'm guessing your motherboard does not.
The Xonar DG and DGX are really the same, except for the PCI/PCI-E interface.
There is even a $10 mail in rebate for the Xonar DGX this month.
The Xonar DGX is really not a lot of money for something that might improve audio quality.