My ultra budget rig is not one I listen to much. I got a Magnavox 1A9217 (Stax SR-3 clone) for $12, with my NAD 3155 from Goodwill for $25, it sounds pretty good.
My sub $100 budget choice is a Yamaha HP-1 out of a vintage speaker amp. For a little more, the HP-1 out of a Starving Student.
Sub $200 is my SR-Gamma, SRD-7, and NAD 3155. This sounds good enough to me that I haven't been tempted to upgrade to higher end Stax. I've heard them at meets, and while they sound better, the price to performance ratio doesn't do it for me.
The headphone I listen to the most is the Yamaha YH-100 out of my Starving Student, about $250.
All of these would sound fine to me directly out of my computer if it's on board sound hadn't died. I currently have an EMU 0404 USB as source which adds $150 to any of the rigs.