Buddy if you are tryin to really get into mastering those speakers can run up to $10,000 US.
Additionally I would meet some mastering engineers and try to gleen somthing from them.
It is a whole different world than mixing...fo sho!!!
If you want some starter software everybody here knows that I reccommend "Har-Bal" check it out on the web or search this site.
But I would start there for a quick start otherwise school, school, school!!
And network,network,network!!!
I really dont think that you are going to be able to "Master" really well with just some really good speakers in the back of your PC though.It takes years(which maybe you have) of experience and ALOT of crappy takes.
Good Luck!! Keep us posted!