Wired devices in a wireless network

Ok so I have a D-Link DSL-2640S-2.X router downstairs provided to me by my IPS. To this, I have laptops and a computer connected wirelessly.

Upstairs, I have my Xbox and another computer, neither of which I have wireless adapter for. What would be the most cost effective, and practical, way of having both of these permanenlty connected to the network?

I've been looking at repeaters, access points, bridges etc, but don't know which will be compatable with my router.

I do have a spare Belkin router lying around that I thought might be able to be used somewhere, but it's not compatable with DD WRT firmware.

Any help into what devices I should be looking at buying would be greatly appreciated!

Thanks.

Edit: I don't want to use Internet Connection Sharing, or have ethernet cables running through the whole house.

 

Powerline adapter or wireless bridge.