For the Airport Extreme option, you could consider having one be an Airport Express unless you need additional ethernet ports you get with the Extreme. It will save you some money.
Keep in mind that you can also use Apple routers to expand the wireless (or wired) network wirelessly. Thus, if you need to extend your WiFi into an area that is not close to an existing ethernet port, you could put an Airport Express (or Extreme or Time Capsule) within range of one of the other Airport stations' WiFi signal and then extend the network wirelessly. I do this with an Airport Express that connects wirelessly to my Airport Extreme as I have a massive masonry chimney in the middle of my house that makes getting a signal in the front of the house tough (the Extreme is in the back).
As to positioning, I would put them nominally as far apart as possible but such that the "outbound side" of their WiFi signal covers the ends of the house (i.e. don't put them on the outer wall of both ends of the house).
I cannot speak how either of your three options with work relative to each other. The only thing that I can see is that I believe a Linksys WRT54G won't support 802.11n. Also, looking at the information for the Hawking is only 2.4 GHz while both the Asus router and the Apple routers will be dual band. Having dual band support many not be an issue for you, however.