Do u mean a normal router with the possibility of attaching an external antenna to it?
will an omni directional antenna of 19dbi do a good coverage of a 3 stories residential building?
What do you think about the Buffalo WHRHPG54? at least compared to the linksys wrt54gs v4!
If you're going to be doing it from inside the building, a large omni will not help you.
The higher the gain on an omni, the flatter the vertical spread, which wont really help you at all.
Use multiple wireless routers and link them with WDS - works for me and my neighbour. I have 2 WRT54G's linked via WDS on DD-WRT fiirmware and they work perfectly.