PnPn-Sai overland cost about 10US$ in a not-so-good minibus, MUST have visum for VN before travel, the other way it now seems can get/buy at border.
these topics are very well covered on the thorntree threads (travel discussions) of the
www.lonelyplanet.com, well-known of the practical guidebooks series.
Besides AirVn there seem also to be fledging Khmer-owned airlines doing this trip, like RoyalPnomPenh [as long as its there] and President Airls, tickets are best bought a few days before, locally from any travel agent.