I agree with djmcmath, try changing your wireless channel from the default. There are 11 channels available for B and G networks, 1,6, or 11 have worked best for me in the past. If you have secured your wireless (no broadcast, wep, etc) then I think you will need to configure your wireless cards for the same channel.
How close is your cordless phone to your router? Try moving one of them if they are too close.
Also if you have a fax line that should proabably have a filter as well.
masraum - The only problem with 802.11a is that is has a shorter range signal and can be easily obstructed. Of course if you live in an apartment or small home this may not be an issue.
Or just wait until 2008 when 802.11n is approved (270 Mbit/s) this may not fix your problem but your LAN will be blazing
Let us know how it goes.