Performance in HD Native

[audiocontrol]

Hello Guys,

I'm brand new to PT after 10 years of Logic.
I just got my new HD Native Card, two 96ios and a 192 digital.
All rounded up by a Command 8.

So far so good, did my first mix today.
I had about 50 Tracks (Audio) and maybe 20 Plugins (Waves, UAudio).

I sometimes got an Error, telling something like "Audio Processing could not be done due to blablabla..... Check your Sync or increase buffer size".

Buffer size at 1024 didn't solve the problem.

Another thing I found out was something like "You're running out of CPU-Power, get rid of some RTAS-Plugins..."

System Usage is at 32% RTAS.

So what's the deal ? Am I missing something ?
ADC is long.

To my System: Mac Pro QUAD Westmere, 8GB RAM.

Can somebody help me ?
Thank you