It's been a while since I've done serious video production (cut my teeth on Premiere and DV using a Pinnacle capture card, all the way back in about '97 through to about 2002... For extra nerd cred, as a kid I was using an Amiga 2000 and Video Toaster back at the start of the 90's), but I think that in this day and age, so long as your HD content is either USB or Firewire transfer, the graphics card doesn't really count for much. It's CPU, RAM and HDD I/O that will count the most during the production and final render.
What are your current hardware specs? And do you know yet which software you plan on using? Production software and codec choice are probably your biggest worries, IMHO.