I got the official first-party VGA Box from SEGA Japan (works with any region DC) for about $40. Third-party native VGA Boxes/adapters/cables are said to be of the same quality, and you used to be able to find them at videogame stores all over here in the US for as low as $10-$15. Unfortunately, DC inventory has mostly been cleared from most places now.
Most VGA Boxes allow you to run the DC through your PC speakers just as you'd think, so no problems there. If you're sharing your monitor between your PC and DC, you might want to think about getting a VGA Box that has a switch for changing between the two (there was a third-party box that did this, but it may be hard to find now. SEGA 1st-party VGA Box doesn't have PC pass-through.)
Anyway, the picture through a Dreamcast VGA Box is best-quality. You'll see detail like you've never seen it from a console game before.