Would Asus Xonar D1 or DG be a good match for expensive headphones?

[begemotik]

Hello everyone,
 
I'm considering to buy Sennheiser HD 25-1-II cans ($300, 70 ohm). I do not have them yet.
On my computer, I have poor onboard audio and some $50 Creative 2.1 speakers.
 
So before purchasing the headphones, I firstly want to buy a PCI soundcard for my computer.
I did some research and found Asus Xonar D1 to be quite what I need, for $70, but with no headphone amp.
Then I found out about Xonar DG for just $40 and with a headphone amp.
 
So my questions are as follows:
 
1) What would be the difference between a $70 soundcard with no headphone amp and a $40 soundcard with a headphone amp when using headphones? Which one will sound better?
2) Are these even a decent choice for $300 headphones? I'm not looking for the best possible amplification, yet will it sound at least nice and not horrible?
3) What about the performance I'd get on $50 speakers with these two soundcards? Will the $70 one without the head amp be significantly better? How would they do if I upgraded to, say, $150-200 speakers?
 
Thanks a lot for any kind of help!

 

[PurpleAngel]

Creative Labs Sound Blaster Z sound card, $80, comes with a headphone amplifier and the same DAC chip (CS4398) as the Xonar DX/D1.