Remember the rule, garbage in, garbage out. Start with a decent soundcard. The chaintech will do if budget is really tight. Otherwise, the EMU is recommended by a lot of people.
Then choose the Headphone amp depending on the Senn model (or any headphone for that matter). As the others mentioned, some cans are easily driven and sometimes do not even need an amp. Some, like the 600, 580, 650 need a good amp for you to enjoy them fully. Some models that i would consider are Gilmore lite, Rega Ear and Headsave classic.
Go to Headfi.org and do a lot of research.