Why do you need it to support Realtek? Realtek is its own audio chipset.
My advise is not to skimp on a sound card. You get what you pay for. You wana spend at least $40 to get anything decent.
i dunno about my mobo.. but google on:
DELL OptiPlex GX620
its the mobo that is in that one.. haven't changed it... (and i have an DELL OptiPlex GX620)