Since I've had my iMac I've noticed that my wifi network locks up a few times a week, it is always fixed by turning off the wifi on the iMac, then turning it back on again.
The network shouldn't be over loaded, it's a Time Capsule with iMac, iPhone, Macbook and Apple TV connected, surely these should all play together nicely!
I've checked and the Time Capsule is at the latest firmware level (7.5.2), so not sure what the next step is.
 iMac is current model 21.5" i3, OSX is at 10.6.6 and software update says everything is up to date.
To clear up unstable the wifi connection always appears to be connected, but if I'm using the iMac I can't view websites, there have also been times where the Airplay streaming has just stopped, so it seems to be the whole network.