When running a wireless network, how much of a hit on performance should you expect versus a wired network?  How much of a hit should you expect by turning on high encryption?

I'm taking a 10-12% performance hit with 128bit encryption on downstream speed, but upstream doesn't take a hit at all.

Any other observations out there?

KingCobra

Wi-Fi
802.11b generally operates no faster than about 5.5Mbps.
802.11g generally operates no faster than about 20Mbps.

Many 802.11b home network typically run at 2-3Mbps.

Performance issues can come from protocol overhead, signal interference, and decreasing signal distance.
Encryption would most likely be placed under protocol overhead.
Sadly encryption is an addition overhead.

If you can try to use 64bit encryption, or disable encryption and go with MAC filtering and hiding your SSID.