If you use a Wii controller as your camera FreeTrack will use almost no CPU. If you use a normal webcam, FreeTrack can use 10-40% of your CPU. If your game needs a lot of CPU as well, FreeTrack might become laggy if it has to compete for CPU time.
However, most people have no problems with this, and if you can play a game at 1600x1080 I assume your computer is more than fast enough to run both FreeTrack and the game at the same time.
Higher resolutions usually mean more work for your graphics card and not your CPU. If you can run the game fine at 640x480 with FreeTrack running, I think you would be able to run it just fine at 3000x1600 too if your graphics card can handle it
I think IR leds are always better if you can find a place to buy them. Any model/brand of IR LED will work. Just remember to grind, file or sand the tips of the LEDs down to 1-2mm above their internal metal components so that you get a wide beam instead of a narrow one. Also, you don't need to put the maximum current through them, and you will reduce their lifespan if you do. My IR LEDs are rated to 200mA, but I only use about 40mA and they work perfectly.
Good luck!