If by "new nvidia card" you mean the 6800, make sure you have a hefty power supply. The card requires TWO molex power connectors and if you're running a PSU of less than 480W, the system will be very unstable.'

You should get a separate, hardware capture card. Make sure that the one you get is hardware and not software-based. If it is a software capture card, it will use the system's CPU to capture and you will get less quality because the video has to travel over the PCI bus to the CPU. If it is a hardware card, however, there is a little chip on the board that does the capturing instead of the CPU. This is better because you get better quality. I recommend a card from Hauppauge.

About your RAM, you would do well to double it to 1GB. With more RAM, you will have less dropped frames in video capture and video rendering will go faster.