[mhh]"Will one of the latest macs and the M100 SW give me similar or better performance than what I have now? "
It will give you MUCH better realtime performance. In Media 100 i your realtime capabilities are limited to ColorFX, Audio Effects, Transitions and static graphic overlays, and all those are done on the P6000 board. In Media 100 i and HD all this is moved to the CPU, and much more can be done this way. Additionally to the Media 100 i realtime effects you also get realtime overlays with or without alpha channel on the V1-V99 tracks (realtime video, e.g. with a chroma keyer and opacity keyframes); additional realtime ColorFX (RGB Curves and Levels), realtime mixing of codecs (Media 100 i 720 and 640, DV, Animation, Media 100 HD all in realtime in the same timeline) and some more. Plus, the rendering of non-realtime FX like Boris transitions are not using the hardware codec anymore. In Media 100 i all video had to go through the hardware codec, which became a big bottleneck for render performance. Now if you render something in e.g. BorisRed in the Media 100 sw or HD timeline the rendering will be MUCH quicker.
If you have a current CPU (DualG4 or G5) you should download the sw Trial version (at the bottom of the page) and try it out for yourself. The performance difference is dramatic.