I don't know about NAS but, I've connected my PC to my C550 via the ethernet port using the Samsung PC share manager application. It plays MKV, AVI, MPEG, MP4 and some FLV files.
I also connected a 1TB Iomega eGo external drive via USB. Works great. There's also the HDMI which I connected to my video card making the C550 a Full HD PC monitor.