3. Don't Mix Wireless B/G with N
Wireless N is supposed to be interoperable with Wireless A, B, and G, but it doesn't always work that way. Some older clients might not be able to connect at all, even when the wireless router or APs are in mixed wireless mode. Then when the older clients can connect, they'll slow down the network.
Some clients may take a heavy 80 percent cut in their throughput. To prevent this problem, consider forcing the new APs to use only Wireless N, and keep the old router or APs around to support the old clients.
Although connections among users on the Wireless N router or APs with Wireless G users will still be bottlenecked by the old clients, the throughput cuts on a mixed mode Wireless N router or AP can be much worse.