The HD cards support SD, yes. SDI in/out, preview out, component if you buy a card with a breakout or the Multibridge. Card does not accelerate render. 422 machine control included. Audio is stereo, always. No help to render times, but remember that uncompressed is a much faster render than any compressed format.
Render generated media to uncompressed, and render again to DV. Uncompressed is faster to render in most situations, because there is less for the proc to do. That's the benefit. The detriment is that uncompressed requires big/fast disk arrays to provide thruput, and big fast arrays cost $$, although the cost has come down tremendously. I look at my 7K$ 420gig Medea and cringe. Not to mention the dual 320 SCSI card and $140 specialty cable to connect it. (per drive)