Nice workstation if properly equipped. If budget is no object, go for a nVidia Quadro CX. If you don't need (for that price) extensive H.264 capabilities, go for either the nVidia 295 or the ATI 4890. Much more value for money IMO. Beware that recently there have been problems with the nVidia drivers and the Adobe suite, despite all the marketing hype. Best bang for the buck is the 4890 IMO.
I can't respond to your other questions, since I have no experience with them. Still using old fashioned firewire.
Just make sure that whatever capture system that it is compatible with Premiere CS4. Check other forums to see how it integrates with CS4.
Like Harm I strictly use FireWire so no experience along those lines. Go with Vista 64. Spend more on the CPU speed, memory and disks rather than the highest end graphics cards.
Vista I won't do - Have been that route twice and not worth the headaches on a machine with capture cards. I have been testing the new Windows 7, as has the integrator I use and so far it looks very good. Loaded the new windows Beta on my backup machine and running it with CS3 - so far no complaints. Will wait for release in couple of months.
I know what you mean. From hearsay I know that RC1 of Windows 7 is much better than Vista. However it is only RC. In a couple of months there will be no choice but to go to Windows 7. If you can wait that long, go for it. If I hear the experiences my son has had with Windows7 and the continuous complaints about Vista (he is an IT pro) I think you are on the right track. Just bide your time because there may still be driver problems at the moment.