Welcome to the forum.
You will definitely get better pictures with higher image quality (IQ) using the K-x then a high-end P&S. In addition, the K-x is well-known for excellent High-ISO shooting. Go for the K-x.
I have the K-7 and I can tell you that 5 fps is excellent. For me, I would prefer definitely 5 fps with high IQ than 10 fps with average IQ.
Another major advantage of the K-x, and more generally of any dSLR, is that you can change the lens(es). And "good glass is worth every penny". You do not need to upgrade the camera if you have other needs. You just use a more suitable lens.
For example, for a boxing match, I would recommend to consider a larger aperture lens (ie low f). Such a fast prime lens may not be in your budget yet, but could be a nice addition in future.
Hope that the comment helps...