I would say give your onboard sound a chance. You can always get a soundcard in the future (for cheaper too), and I get the feeling that you'll be more than satisfied with the sound coming out of the mobo.
As for which set of speakers, Logitech simply makes good ones. the 5300s are basically the best deal on the planet, more powerful than the X-530s, and much much cheaper than the 5500s. I really wanted one, but I ended up buying the X-530s because it's so cheap (under 50 bucks), and the monster of a subwoofer on the 5300 is just too huge. I've actually been very pleased with the X-530s, and I've even thought its relatively small sub is more than enough for me.
So in conclusion, I'd say skip the soundcard, and as long as you're not some crazy audiophile, give the X-530s a shot, and you've saved $100s that you can use on the important part of the computer, such as being able to get a bigger brighter (maybe widescreen) monitor!