I am thinking of getting an external usb soundcard and a pair of good headphones for my laptop.
Should I get a M-audio Transit or sonica theatre for the soundcard and for the headphones I was thinking of getting one of the 5 shortlisted below:

Sennheiser Hd 280 pro, 555, 515, 500, 570

WHat do you guys think I should choose for the soundcard and headphones?

I have the M-Audio Audiophile USB. I think the Transit, Sonica (do you want the Theater for headphones?) and Echo Indigo (if you'd consider PCMCIA) are all good options.

Haven't heard all your selected phones, but on sound quality alone, I suspect the 555 would be most peoples favorite. You're okay with open phones for your uses? Only considering Sennheiser or opening it up to Grado, Audio-Technica, etc. Depending on use, the Alessandro MS1 (also open) and AT A500 (closed) are also good choices in your price range.

Also you may want to check out Head-Fi for more headphone info.

Just a thought:

Try to get a soundcard with a game port included: Saves you from big problems when you start working with MIDI or Joysticks!
Had that problem recently!

JD

Well, it really depends which features suit you best yet while staying within your budget. Since I don't know what either of those are I'll give a vote for the Sonica which is basically the same as my Revo except for the 192khz ability iirc and the HD-280s which I also own and love. Together they cost me $170 so imo it was a very good deal considering how much use I get out of them daily.

If the review by ixbt.com can be trusted, sonica theater isn't that great - see the RMAA results. Further, the output level will be quite low, and it will barely be able to drive the headphones you're thinking about. Make sure that you'll be able to return it if you're not happy with the performance.

I decided to go with an audigy2 nx. Surely, it resamples, but foobar2000 can take care of that... and its headphone output does a very good job of driving my 300 ohm sennheisers.

As for the headphones, see if you can get Sennheiser hd580. You should be able to find them for ~150$, if you look hard enough. I have them, and I love them.

I was thinking of getting the hd555 or the 280 and for the sound card i want something that doesnt require an external power supply. COuld you guys recommend another sound card that could power something like the hd555 or 280

Are you sure?
I recall getting in a small argument (in which I was wrong) over whether the Extigy or Audigy 2 NX/MP3+ resample to 48KHz. The external cards do not use the EMU DSP, and the Extigy at least for sure does not resample 44.1KHz content, meaning the 2 NX probably doesn't either.