I run
WLAN Optimizer when playing on wireless, its main feature being its ability to disable background scanning. I see an overall latency reduction of only 50ms or so, but the elimination of spikes is very nice. I run Windows 7 btw.
You can run an extended ping test (e.g., start > run > ping -t 192.168.0.1) to your router; anything over a couple ms round trip could be indicative of an issue. A side by side ping test to your router using both a wired and wireless source can also be a helpful in isolating lag.