You may not need that 24 bit resolution now, but that's not to say you won't. Any card that purports 24 bit resolution very likely has much better hum and noise performance than a 16 bit card and that may be of value some time down the road. The extra dynamic range may come in handy when the real world tries to bite you in the arse.
Say your digital audio track from a shoot is bad, perhaps from the camera man clearing his throat ever so softly at the same time one of your actors has turned away and all you're getting is his voice bounced weakly off a wall right at a critical part of the dialog. You can't digitally edit that out and voice-over later without making a very noticable 'hole' in the room ambience. But you have an analog recording on another backup camera, and it's output level is low. That extra dymanic range may let you amplify that to a useful level without introducing the sound card's own hum and noise in the process.
Fav quote - "Experience is whatcha don't get 'till ya don't need it no more."
System - Athlon 1.4GHz, Win98, Hauppauge PVR250 receiver and compressor.
Software -Magix Movie Edit Pro 10, Nero 6 + NeroVision Express, Moho 4.61, PSP 8.1, Bryce, Quicktime 6.52 pro, Goldwave 5, DVD-Lab.
Cameras - Panasonic GS9, Canon ES8400V, Canon EOS D20 and Canon A70