My capture system (WinXP) hsa 1GB of RAM, 1GHz Athlon CPU. It is used solely to capture and store.
My processing system (WinXP) has 1GB of RAM, Athlon XP 2.7GHz CPU (2100+ @ 2700+ rating, overclocking). It is used to clean up and then burn the resultant image onto a DVD disc.
The reason why I have two machines is simple: I hate having to put more software on a computer than is necessary. Extra software takes up disk and registry space which in turn affects performance and available disk space. It may be overkill, but it works for me. (besides, I like keeping the digitizing computer on its own clean electrical channel to prevent interference showing up during the digitizing process...)
I'd recommend a separate video card and capture card (in the computer industry, ANYTHING that promotes itself as "all in one" has all in one but all is low quality crap). Any video card will do. But for a capture card, especially if you're archiving store bought tapes, the capture product and/or driver has got to be able to ignore the Macrovision signal or else it is useless. Macs don't have the macrovision intercepting software. ATi cards DO, making them useless to me.
www.videohelp.com (foremerly ww.vcdhelp.com) has much more on capturing.
For more on why I prefer Pinnacle because of macrovision:
http://www.aluminumstudios.com/digitalvideo/advanced/dvd.html (do a search for "Pinnacle" and you'll get taken right to the money)
BTW: I don't want to spend $3000 for one Mac when two $600 PCs will do the job just as well, and for their own dedicated purposes. It makes me look more cool too and I learned more concepts in the process, such as networking. (useful if I wanted to find a job in IT about 3 years ago.) :D