Actually you dont really need an extremely fast CPU, a ~1.66Ghz or so is perfectly fine because you usually record in video formats that doesnt use "very" complex compression algorithms. Analogue sources (VHS, Video8 etc) should be recorded in HuffYUV or similar, my old AMD XP 1800+ didnt even break a sweat recording in full PAL resolution. DV on the other hand should be transferred using firewire to avoid unnecessary conversions. It pretty much works as any other transfer over firewire which means low CPU utilization in general.
What you do need though is lots of space, HuffYUV is around 25-30Gb / hour while DV is roughly 12Gb / hour. At least have a separate disk for video editing. Preferably a fast one such as Hitachi T7K500 320Gb or such. Video card wont affect performance noticeably unless you have a very old one. As long as it supports YV12 and 32-bit color space properly which pretty much means anything that's been made the last 4 years. Old integrated graphics solutions my be a bit quirky in some cases. As for RAM it really depends on what you want to do, if you want to record and then do some simple editing in VirtualDub or such 512Mb will be fine while 1Gb is recommended if you want to fire up Adobe Premiere or such. I belive the Uleads Video Studio (or whatever they call it) has a lower memory footprint but it is a bit scaled down when it comes to features although I doubt you'll notice a difference.
CPU speed will mainly affect rendering speed, both formats are very easy to decode while converting and applying filters isnt as easy. Dual Core is not a requirement, most encoders doesnt even use SMP. XviD for instance does support although you get some weird 70/30% work load which doesnt helt much. H..264 doesnt support AFAIK and some MPEG-2 encoders such as TMPGEnc Plus and CCE. You have a few free alternatives such as QuEnc which doesnt support SMP so it'll only use one core. As for capturing card I'd highly recommend you to get a card without a hardware MPEG encoder because software cards are much easier to work with in general and works just as good if not even better... AverTV 777 DVB-T or A16A should be a good choice if you want to have a decent card without shelling out lots of money. They also support (as the name implies) DVB-T meaning that you can watch FTA-channels (Terrestrial) and record without any conversion at all. Remember that video quality also highly depends on type of connections, S-Video is much better than composite so have that in mind when you hook it up.
The difference between A16A and 777 is FM-tuner and "cable tv".
777 only supports DVB-T + Composite + S-Video input.