...sorry been away a few days. To answer the question, I would say most definitely a good interface is a must when recording from hardware to a computer. Any computer. I lucked out in that I bought an 001 so my software came with a decent interface. I still bypass the analog ins though and use the Rosetta to convert A/D via SPDIF.
But I would say most average stock computer soundcards are not properly equipped for quality audio recording. That's IMO of course, and I'm sure as always there are exceptions. If money is a concern I would cop the most affordable quality interface with the least amounts if I/O's (ya know like 2 ins, 2 outs + SPDIF). Multi outs and all that is not necessary without a mulitrack console setup.
Ultimately, I would get a converter. The Rosetta 200 is nice because it has A/D and D/A so its quality in, quality out. I got the old plain Rosetta which is just A/D. At the very least look into digital conversion, it'll boost the quality of your audio, especially when recording to software and what not.