So the power of the PC could not be the bottle neck.
I'm not sure about that. Only 2 GB RAM when capturing over SDI could very well be a serious bottleneck. Your hard disk setup could be another. SDI can go up to 1.485 Gbps data streams and a lot of systems get overwhelmed by this data rate. You may need a 4 disk raid0 array or better to handle that stream, at least 200 MB/s sustained write rate.
Thank you very much fpr your answer, I have forgotten to tell you that I'm working with an external Lacie Raid Array "Biggest 2S2 with five discs in Fast mode Raid0. The device is connected via a special eSATA controllercard to the Raid. The controllercard come also from Lacie and is tuned to work with the Raid. So I think that the performance should be enough. But Memory is eventually less, you've got right. I try to upgrade to 4GB Memory.
And what's about 4.01 and 4.1 version. Motu said that the current driver still running under 4.01? How to downgrade without reactivating the software?
I have heard of and read about the Motu, but never worked with one. I think you best follow their advise on using 4.01 or 4.1. Sorry I can't help you with that.