The unfortunate story is that there is no one magic "Just add this plugin and you sound like a CD". At least if there is, I haven't found the link to buy it and it would probably cost mega-bucks anyway......
So said, Sonar XL does come with quite an impressive range of plugins to allow you to perform the basic, EQ, compression and "cool" type effects. These can go a long way if you spend the time to get to know them and how they operate.
From a mastering perspective, I was quite impressed with the range of functionality and flexibility in Izotope's Ozone and it looks as though the T-Racks Mastering software also works reasonably well, but is harder to control.
Other people swear by things such as Sound Forge and it's included array of tools which can be used for mastering purposes.
My advice would be to play with all the included plugins until you know exactly how they work and what impacts they can make on your songs. Ultimately though, the old maxim holds true, you can't polish a turd into a diamond. Get your original recordings as close to perfect as you can as this will reduce the amount of work you will need to spend both mixing and mastering.
Timeworks, the people who make the built-in compressor and EQ with Sonar also have quite a nice mastering compressor.
Also - make sure you check out demo versions - most of the big companies make them available and nothing beats testing these little beasties out before you commit the cash...........
Waves make a good plug-in collection - but again, it's how you use the tools not the tools themselves that make the major difference.
Let us know what you decide!
Ciao,
Q.
