If you are using a capture card, there will be a difference as you are compressing the footage. Are you judging this on an external NTSC (or PAL) Monitor or TV? You cannot judge by the computer display, no matter which on you have…
Why not go straight in via firewire and go in lossless? If you go in via firewire it is a simple data file transfer and the footage will be EXACTLY what you have on tape…