nstreme is a MT proprietary protocol that can use a polling technique to allow clients to talk with the ap. Think of it as a token ring, when a client recieves the token it gets to talk for x amount of time. It helps avoid collisons and speed up throughput, but it also eats up CPU cycles. It also allows for full duplex communications with Nstreme2. You can read more about Nstreme in the MT RouterOS manual.
All clients must be MT and have Nstreme enabled in order to participate in an Nstreme enabled network.
And if you look around on MT website you'll see they have 802.11a/b/g cards for sale, atheros and prism based. You can also get these from one of your local wireless distributors, try google.com.