[markzi]

I'm getting a pair of Sennheiser HD 598s,  and I want to use them on my Desktop. Do I need an amp to get good quality? What determines if an amp is needed? The sound card? Or the Power Supply? Do amps make that much of a difference?
I know I'd probably need an amp if I were to use them on my iPhone, but I am concerned about the quality on my Desktop.
I know, stupid questions, but I'm new at this stuff.

 

[Spareribs]

 
In the world of Head-fi, yes. An amp is requisite. 
 
However, in the real world where people are less picky (or less crazy?) an amp is not needed if it sounds good enough. 
 
I would buy an amp and see. It is really the only way to find out unless if you can borrow one. If you end up not seeing an improvement, you can sell it. You may lose a little money but you will gain personal knowledge about your taste. I had questions like you and to really find out, I actually had to spend money and buy stuff. I personally do not like to rely on other people's opinions because even smart people can be incorrect.

 

[Dubstep Girl]

yeah, definitely get an amp.

 

[PurpleAngel]

The Sennheiser HD-598 are fairly easy to power, but built in sound on the motherboard will not bring out their best.
Should be able to find a headphone amplifier (with DAC?) in the $65-$100 range.