Infinity has always been my speaker brand of choice, at least as far as mids and tweets go. I would have to say that you have made an excelent choice. The kappa series are some of the clearest speakers you can get, and if I buy more speakers for my car, they would be kappas. As far as a sub goes, I would recommend JL Audio. That's what I have, and it's amazing. I got one of their small angled boxes with a 10 in it, that fits perfectly in the trunk. I can't believe so much sound comes out of that small of a box. It's unbelieveable.

When I went to purchase my sub, I spent a couple hours down at one of the local stereo places in town, trying out all of the diferent boxes that would fit in the trunk. The JL Audio 10" enclosure sounded by far the best, even versus larger ones. Some of the bigger boxes had more low end, but the 10 has good low mids, a nice hard hit, and good lows. For what I wanted for my system, it was a perfect match.

If you can find a place that is willing, I would sugest a similar trial run with various boxes at your local shop. Choosing the right sub is going to depend on how much space you have, how much power you're running, how loud you listen to music, etc.

Good Luck!

-Update-
This is the exact sub that I have. It fits in the trunk, facing backwards, and the angle on the back of the box matches perfectly against the back of the seat. (at least in my car...)