If you're shopping for a semi-pro/pro system, you might wanna have a look at
Promax. They have pre-configured systems for editing. It might save you a lot of headaches later.
But if you just want to do some personal stuff, a cheap OHCI-compliant firewire card or a ATI ALL-IN-ONE will do the trick.
In any case, start building the system from the ground up: first choose which tape format you want to support (dv, betacam, vhs, etc). Then pick your digitizing-I/O hardware and nle software. Once you've done that, check with the manufacturer and/or users (mailings lists and forum) for recommended hardware configuration and STICK WITH THAT.
Which format do you plan to use?