I guess there is a difference in what people mean when they say mastering. Knowing what I know now, buying a great wav editor would be the best place to start. T-Racks and Ozone are both very good at what they do, but if you need crossfades, meta data...and all of that. A wav editor with VST support is the way to go.

If you are going for Ozone 6 at $250, you might want to think about stretching it a little to $335, you can get the Izotope Studio Bundle (Also mentioned by metellus above) which includes Ozone 6. Alloy 2 and Alloy 2, Nectar 2 Production Suite, all very useful tools.

I'm relatively new at this and Sonar is packed with stock plugins, but I couldn't get them to bring a final mix close to what I was hearing on professional reference songs or to the professional quality mixes you find here and at other music sites where skilled studio gurus reside.
 
I first picked up the T-RackS Classic suite from IK because it was on sale and I was looking for a less expensive solution.  Some folks use it for mastering.  Those IK plugins are useful, but I still couldn't get the quality I was after.
 
I bought Ozone 5 during this past year after recommendations from those same gurus - to my ears the difference was immediate and immensely better.  Instead of loud and harsh, my mixes became fuller and smoother.
 
I have much to learn, but Ozone makes that process much more fun and a whole lot less frustrating.  If you have the bucks, go for it.  Good luck.

bitflipper
olemon, make sure you get back over to the iZotope site and download a copy of their Ozone Mastering Guide pdf if you haven't done so already. It's an excellent resource, even for those who are not Ozone users. 

I do have it, though I haven't studied it like I probably should:(  

PlatanoMuse
Seem interesting, but dongle required..

Just had a quick scan through what has been mentioned, The Izotope stuff, Ozone, Alloy 2, Nectar, don't require a dongle, neither does the T Racks stuff or most of the other stuff that has been mentioned. The Slate Digital stuff does require a dongle, iLok2 however, but for me after playing around with it on a friends setup, well, it's ok, but I prefer what I have, am quite happy now with the results I can achieve.

Landr at Mixgenius if you are more interested in writing music and not dealing with the complicated job of mastering.