If all you are trying to capture is material from a VHS VCR via an NTSC Composite Video plus Stereo Audio line, you do not need HD capability. The resolution inherent in an analog NTSC Composite Video signal is roughly 640 x 460 or maybe 800 x 600 in computer video screen terms. Virtually any analog video capture card that does NOT claim to provide HD will get you that resolution or higher in the file it captures. Anything more than that is a software interpolation that exceeds the quality of the original signal.
Now, IF you are looking ahead and thinking that you may want HD capture capability in future, plan you buying that way. But you don't need it now.
You could consider also a more complex and expensive card for other added functions. For example, some of the good TV tuner cards include video capture capability, with dedicated inputs for that. If you might want HD capability for the future, watch carefully for the type of input port. An input which is a standard cable TV Type F connector and requires the signal as a standard TV signal on channel 3 or 4 can never give you high definition - the old analog TV signal system does not have that bandwidth. You would need an input of the video signal itself, usually on an RCA coaxial cable connector like a Composite Video signal, or an S-video or Component Video connection that specifically says it CAN accept true HD video signals. In many of these cases you will end up connecting your video signal to the video capture port, and the stereo audio signals separately to the audio inputs of your sound system.
I like your thinking get a dual TV tuner card, if I understand you correctly this will allow me to
1 Capture my old VHS a/v tapes or a/v for any other analog source that I have connectors for.
2 Connecting my Time Warner Cable to SD TV tuner will let me watch on my PC same basic channels that I now see on my analog TV
3 Connecting my cable to HDTV connector on PCI card will let me watch HDTV on my PC
Questions for you
PCI connector interface types, picking the right one.
Do you have any PCI card/software suggestions
You are right on all three items, but with a couple of "watch for ... "'s
1. Tuner cards come in several designs. Some have one tuner with limited capability, but most one-tuner cards can handle all signal types. These could include ATSC digital TV from OTA broadcast transmitters in either SD or HD formats, Clear QAM (an unencrypted digital signal) on cable, and analog old NTSC broadcast TV on cable. These are all unencrypted. Be aware that digital TV on either OTA transmissions or cable services may be either older SD or any of a few types of HD, so don't assume all digital channels are HD. Likewise, don't assume your basic cable channels are all SD and the extra-cost items are HD. Ask your cable service for details. The encrypted pay channels on TV, whether they are SD or HD, are done a couple of ways. The straightforward way is to let your cable unscrambler box do the decryption and then feed the best signal type you can from it to a set of inputs on your computer, whether they are on your tuner card or otherwise. The other that is just on the market in the past year and changing - it is called the CableCard system. It is a new encryption system for cable operators. The cable company needs to be converted to this, and you need a tuner card with this feature. Then you pay the cable company for a little key module that fits into your special card and it will allow you to tune in their pay channels. Consult your cable company BEFORE going this route.
2. The fancier TV tuner cards have TWO tuners in them, plus an FM radio tuner in some. BUT these come in two configurations. Some have two input jacks on the back for the TV cables (or 3 if there's FM), but each is dedicated to only one tuner with only part of the capabilities. More commonly one tuner will handle ATSC digital TV from local transmitters, and the other will handle all the cable stuff - Clear QAM digital and analog. The other configuration has two fully capable tuners inside sharing the one input connector. Each tuner can handle all signal types, so it really is possible to tune in two different signals at any time, no matter what their source and type.
As to interface types, I am not sure. I suspect that PCI Express is a better choice than PCI for bus speed, and I see that many tuner cards are specifically PCI Express x1. Now, it seems to me there are not a lot of those x1 devices around so that slot in your machine may well be empty, with no likely other use in the near future. So using it instead of something else might be a good idea.
On the choice of card, I think your best source is to start reading and asking in the Graphic & Displays - TV/Video Cards forum on Tom's. You will get people with real experience, and you will catch up on the latest developments in this field, like sources and prices and deployment of the new cards. I know that Hauppage has some good units, especially their top-of-the-line full dual tuner one. The CableCard tuners that can decrypt signals on cable are from other sources, I think. And there is a lot of buzz now about the latest CableCard units from a company called Ceton that will have 4 tuners, and a 6-tuner version coming later.
One thing to check: to use most of these you will need to be running Windows Media Center. It's built into the new Win 7, but was add-on stuff with previous Windows.