Wow... this thread switched tracks to audio enthusiast town quickly.
DeadOfKnight, I was in the same predicament as you. There were people telling me to buy bookshelf speakers and an amp but I had limited space.
While it may be true that bookshelf speakers + amp is better, I picked up a set of Klipsch ProMedia 2.1 and couldn't be happier for the $100 I spent on them (cyber monday deal). I don't really like that there isn't a switch to shut off the set though but that's me trying to be critical. FYI: at idle the setup sips ~8.5W of power whereas my old setup did about ~4.5W (with off switch). I personally didn't want to pick up a 5.1 set because it sounded like more of a hassle and, again, space constraints. Anyway, music, videos and games all sound crisp and clear.
If you're not an audio enthusiast (like me) you're going to really like a quality 2.1 set of computer speakers. Plus, if you do want to eventually spend the money on an "enthusiast" setup you'll get the chance to see how much of an improvement it is over "enthusiast" computer speakers.
Most bookshelves will give you better accuracy, but as you say, this may not be what you're after. I personally find most computer X.1 systems (with subwoofers) a little unappealing due to very sloppy mid-bass (and little actually bass below 40 or 50 Hz). I owned an original Klipsch ProMedia 2.1 that, at least at first, could be tweaked to sound pretty decent. The Logitech systems I've tried (Z680s and Z2300s) sound absolutely terrible with their sub. The satellites on the Z680s were pretty decent at what they did, but the sub, and integration with the sub was just abysmal (though it was loud).
As far as size goes, you don't need to large speakers. However, if you go with smaller bookshelves (especially with woofers below 5"), you'll likely be missing any sort of bass punch. If this is important to you, and room is an issue a computer subwoofer system may be your best bet. If accuracy is more important, you can get smallish bookshelves that still sound good.
One example are the Audioengine P4s
, which can be had on sale for $200 or a bit less for the pair. They are only about 6" wide by 6" deep and about 9" tall. Build quality is great. I have a pair connected to a desktop T-amp, and it's great for nearfield or moderate sized room. These, together with the DTA-100a (and I'm sure they'd be great with a lower-powered amp for nearfield, such as this
, as well) should get you an awesome 2.0 system for about $300. And you can always add a subwoofer later.