The upgrade is at the MOTU site for 4.5 owners to download. It is supposed to allocate your resources better, thus taxing your system less, making it more efficient.

However, if you run Virtual Instruments, especially Spectrasonics Atmosphere, beware that it will not work right with DP in 4.51. there are also reported bugs with serious latency using other VI, including Motu's own mach Five. I have not upgraded from 4.5 myself as I use Atmosphere quite a bit. I understand that the problem is with the AU and an effective workaround is to use Atmosphere in vst mode with a vst wrapper. For now, though, 4.5 0wners upgrade at your own risk.
I myself am going to wait til they work out the bugs... :D