Cant believe you missed the #1 cause of wireless issues: Wires! All cables that have power running through them give off electromagnetic signals that will interfere with your reception, especially if you are at the edge of your wireless range to begin with. This is especially true for desktops crammed in little cubby holes. Make sure there is ample space (6-12 inches) between the back of your tower and the wall or cabinet. Try to keep the rats nest of cables away from your antenna. Also, metal cabinets, drawers, etc are going to act as shields, so use wood furniture around wireless devices.
Also, putting your antennas parallel to each other (either vertical or horizontal) can create limited range or dead spots. Set your antennas aprox 90 degrees from each other for best performance. Lastly, don't put your wireless router in a little closet somewhere. Hide it on top of a bookshelf or something. I understand not wanting to look at the thing, but enclosing it will change performance.
Lastly, wireless devices affect each other. The more wireless devices you have the less performance you will get. All stationary devices (the network printer, TV, media PC, PS3, Wii, xbox, blue ray player, desktop PCs etc) should all be wired if possible. They will get better performance this way, and your wireless devices (laptops and network capable cell phones) will have free air to work with.