One. Turn off 802.11b if you don't need it for any of your devices. It's one of the biggest killers of a wireless signal for 802.11g/n devices.
Two. Change DEFAULT RATE from Auto/Default to the highest possible without connection loss for devices farthest away from your AP.
Three. I assume that you're on Windows. Download
Inssider and monitor wireless signal from neighbors that enter your house. If everyone's AP is on channel 6 (default for most routers), set yours to channel 1 or 11.
Four. Replacing your router most likely isn't going to solve your problem. You're going to have to either:
- redirect your antennas (it can make a big difference since they're omni directional)
- move your router to another part of your house
- mod your router with higher gain antennas (get ready to break out the soldering gun and plastic epoxy for your particular model)
- setup repeater APs to extend wireless signal for devices that have problems connecting
Five. CHANGE YOUR FIRMWARE to one of the open source ones (OpenWRT/DD-WRT/Tomato)! Frankly, most manufacturer's firmware for residential grade equipment sucks.