You have a nice new set of headphones, and I hope you're pleased with how they sound so far. They will last for years, therefore I don't think that it is urgent that you immediately $pend more. Unless you have a bunch of cash laying around, take your time and get to know them.
Having said that, they will improve a lot, should you eventually decide to invest in better gear to run them.
Regarding x-fi settings, try this:
Audio creation mode, set the master sampling rate to 44.1, bit matched, 2 speakers. Muting the inputs is probably a good idea, too.
I find this, combined with foobar using kernel streaming output, gives me the most 'honest' sound of what the x-fi can do. There are others who prefer other modes/settings, and you should try them all, to see what you like best. That's what counts.
Regarding upgrades, I think you'll have to provide a budget with your question, or you'll see pandemonium ensue. Just be sure to remember to upgrade the cable for your Sennheiser's, too. I did that with mine
before external dac/amp, and the difference I got from my x-fi was huge. The cable that they send stock is ridiculous considering how they sound with a better one.
Welcome to Head-Fi, and happy listening!