Testing network performance

[Karamano]

Disclaimer: new to linux, so sorry for asking these questions.

I am unable to stream smoothly video files over the network. The old school regular quality dvd rips stream ok (barely), but anything larger than that (720p and so on) is not even close to running smoothly, buffering 20-30 secs for 2-3 sec segment. Trying to stream files form a Win7 x64 computer. Tested streaming protocols SMB, NFS and UPnP- all stutter. Files are .MKV and .M4V type- anything over dvdrip quality stutters.

Hooked via LAN to a router 54Mbps (G). Tried wirelessly as well with my Samsung phone which can tether at 100Mbps (N) and wifi dongle Edimax EW-7811UN- same stuttering.
Internet streams, even HD ones like BBC, Hulu etc. stream smoothly.
Using my weak and limited Sony SMP-N200 streamer everything streams perfect over LAN and Wifi.

I thought the best way to begin approaching this problem was testing the network performance on the Pi with something like iperf (as google brought up), but unable to get the command working via SSH. Adding 'sudo' before it did not help either: "iperf: command not found".
Can someone explain what code I can enter to test the capabilities of my network to stream/transfer files so I could begin to look for solutions why my Pi is unable to stream decently?