I am making a list of things to get with my jvc hd cam and I have decided that I am going to buy a capture card so I can shoot in HD and down-convert to achieve an stronger source of video than dv... ok with that said I am guessing that I will need a separate audio capture card ... or do I any help will be greatly appreciated

I'm not sure I understand exactly what your plan is.

Are you planning to capture analog video and audio using the built-in downconversion of the camera? I would not recommend this as NTSC and PAL will only output via the composite out and not component. Also, this would add additional and unnecessary DtoA and AtoD conversion.

If you are planning to capture HD from the analog video & audio outs, then there is a distinct advantage if you are not recording to tape first. The thing is, you won't need a separate audio digitizing card because most of the leading analog card manufacturers (AJA, Blackmagic, etc.) include audio on their cards.

If you are recording to tape and then digitizing later, I see no real quality advantage to capturing from the component analog outs other than real-time workflow at 60P.

I am going to purchase a Blackmagic card so I can down-convert, because the final product needs to be in SD not HD... but I want to shoot everything in HD because the video source is stronger.

I am going to purchase a Blackmagic card so I can down-convert, because the final product needs to be in SD not HD... but I want to shoot everything in HD because the video source is stronger.

Well, in that case, you won't need a separate audio capture card.
Which model of decklink are planning to buy?

I'd like to follow-up on Tim's advice. A special soundcard won't help much during the ingestion process, especially if you're working with the AJA or Blackmagic cards.

A high quality sound card helps with the audio mixing/editing. If you're planning on really tweaking your audio, 5.1 mastering, mixing sound FX, etc, than a good sound card will help. I'd check with your NLE package and confirm what is compatible, but I hear great things about the M-Audio cards. I don't do any fancy mixing, so my integrated sound card works fine. I'm not against any SoundBlaster model, but they sure are resource hogs and full of bloatware.

Hope this helps.