Mark, you don't just need a better access point, you also need better antennas. Linksys routers blow chunks because the antennas aren't connected to standard screw-in connections. You want something that takes standard oversize antennas, usually the "SMC" type.
Your Linksys router at best has a pair of 3db antennas. Stick a pair of 7db or 9db on there and whoa, no more troubles.
Personally I like Hawking wifi gear - everything is set up to allow massive oversize antennas. Last I checked Fry's Electronics has the whole Hawking and Trendnet product lines. I set up an extended range wifi install in Tucson recently using a pair of Trendnet 7db antennas on a single base:
http://www.trendnet.com/products/proddetail.asp?prod=140_TEW-AI77OB&cat=68
...plugged into a Hawking N-class access point. Punched through multiple 3ft-thick interior walls in this ancient adobe building :). Whole thing cost $108 total with tax :). I could have done a pair of 9db antennas plugged straight into the access point but this was a cleaner install, the Trendnet "base" could be bolted to the wall fairly high up, which compensated (I think) for the slightly reduced raw punch.
You want omnidirectional antennas rather than directional types for your needs!
You MIGHT be able to find adapters allowing oversize antennas on the router you've got, but it's still an N-class device. I'd jump to N-class plus standard connectors to oversize antennas.
Jim