Right click My Computer>Properties>Advanced>Performance settings.
I notice you didn't mention a soundcard, if your using the stock soundcard that came with your computer then you definitely need to upgrade. Eventually you're going to need to upgrade your pc also. On the low end the Audiophile 24/96(150.00) is a good choice. If you got money to spend I'd reccommend a MOTU 828II fire wire interface(800.00) or the new Tascam FW884(1300.00). Both of these would allow you to get rid of that behringer mixer and go digital all the way.
As far a your recording technique of recording everything into SF and then importing into Sonar, I'd say you're doing it the hard way, just record directly into Sonar. If you get a new audio/midi interface you will be able to sync up all your gear which will make tracking much easier. You will be able to track each individual track from your MPC and Triton and mix it in Sonar, the difference in sound will be evident once you learn how to use all of Sonar's tools.
As far as sampling in SF instead of directly into the MPC, whatever works best for you is the best way to do it, I sample the same way you do most of the time. You'll notice a difference in the quality of your samples when you upgrade your soundcard.
When it comes to outboard gear I'd recommend you get a good mic pre. I good mic pre is gonna run you from 500.00 to 2500.00 bucks, just remember that when it comes to recording equipment you usually get what you pay for so get the best you can afford. On the low end I'd recommend the Grace Design Model 101(600.0) or the Summit Audio 2BA-221(600.00), if you can, get two of them and you can run all of your gear through them and really make your recordings sound great. Don't take my word on what gear to buy though, do some research on this stuff and see what looks good to you. Check out some of the recording magazines, they always have great tutorials and reviews of gear.
Let me know if you decide to upgrade your soundcard and I'll let you know how to hook everything up . Good luck!