Audio is fairly easy. If you already have a turntable hooked up to an amplifier, you can take a line out (usually TAPE OUT) and connect it directly to the line-in on the iMac. You can use Garage Band to record, or something like Amadeus.
If you don't have an amplifier, which means you only have mic level, you need a pre-amp. Griffin's iMic has mic level input, and comes with Final Vinyl - which might be worth the price of the iMic as it provides RIAA compensation, and pop/hiss filters. Otherwise, you will need a pre-amp with specific phono inputs (with RIAA compensation), or just the pre-amp and apply RIAA later in software.
For video, it's a bit harder (as there is no video-in standard on Macs at this time). A lot of Mini-DV video cameras have analog video inputs. If they can do straight through to Firewire, then you can hook up your VHS player to the camera and use it to input directly into iMovie or Final Cut. Or, if you want to go straight to DVD w/o editing, you can use iDVD's One-Step DVD feature.
If you don't go the camera route, then there are several analog video to Firewire converters available. They will also input video directly into the previously mentioned apps.
Have fun. Reminds me I have to get my LP's out of storage and put a new belt on the turntable.
Edit: by the time I hit "save" a couple of posts had already brought up an even simpler method for video, at least, of using a consumer DVD recorder. I guess my first inclination was you wanted to do some editing in iMovie or FC. Although, you could input a DVD of your source back to DV, but you will lose some quality since this would add one iteration of compression (DV to MPEG2 to DV), whereas the camera or other firewire converter goes direct to DV.
