Building your own requires a wireless device with an external antenna. While I never built my own antenna (for 802.11, I have done it for VHF and UHF though), if you get a usb wireless adapter or a PCI device with antennas you can build reflectors to increase signal strength but reducing the beamwidth.
I had a belkin G USB stick it came with a 3' usb cable and a port to stand it up on your desk. I experimented with some shapes around the house (piece of a floor lamp with antenna sticking through center) but generally the best reception comes a V shaped design maintaining wavelength sepcification. Most 2.4GHz run on quarterDipole size as long as you stick wit ha mutliple of that you will be okay. Some prefer quarter, some prefer halfdipoles. I never found much of a difference in my experiments though.
Here is a good read with some links
http://martybugs.net/wireless/rubberducky.cgi
If you are going to go with a router you can build parabolic dishes for hte antennas, get a DDRWRT capable router that allows increasing power... DDWRT should also have wireless bridge functions. But I am not 100% certain of that.