The internal scopes eats up your performance – it’s not worth using if you’re after real time playback.
I have an early 2009 with the basic 2.26GHz setup.. 6GB RAM, GT120+GTX285, Decklink HD Extreme 3D and RocketRAID eSATA with a 4TB array in RAID0 to gain as much speed as possible.
The few times I hit the wall in regards of performance is with RED RAW at medium or full quality, 2K and 4K DPX. SD and HD (24, 25, 30 and 50fps) in any format has been no problem.
I sometimes find it necessary to switch on and off the proxy mode because the controls get a bit slow over time. Switching proxy on/off fixes this though.