If you are looking for a headphone to 'build around' then the HD600 is a good choice. They will scale very well. The better the components you put behind them, the more the sound quality will jump up. They're not really going to sound anywhere near their potential out of an ipod, but they are listenable. I compared them to the Grado earlier, and with headphones, a lot of what you prefer to will depend on mood. These are great 'relaxing' headphones with a very smooth sound and will only get better as you upgrade your rig.
A relatively inexpensive amp that would go well with the HD600s is the Darkvoice 332 if you can find it used since it is discontinued. Or an EF1, which I have. It's not the 'most synergistic' match, but it performs decently with the HD600s. You'll probably need a DAC to get sounds to play out of the computer.
What I'd recommend you to do is get an EMU-0404, which is around $200. It's primarily a DAC, but also has an amp component. Just use the amp out of it until you can afford a decent amp later on. It won't sound 'as refined' as a standalone amp, but it will definitely hold you over until you can get something else.