If I remember correctly from one of my Best Buy friends, if you pop the hood of a Linksys, DLink, and Netgear router, you'll find they're all using the same exact chipset mounted on very similar boards. The main difference being the packaging and case. Which to me means you can go with either of these brands and do just fine.
The Linksys WRT54G has 3rd party firmware available. I'm not sure about the other flavors of Linksys (there's a newer version of WRT54G that is firmware friendly as well). The older models can have their firmware upgraded as well, but I don't believe there's any 3rd party ones available for them (yet). I have a cheap B version which I've upgraded the firmware on already, but there's no 3rd party ones available for it.
As for signal strength, I think you won't find much of a difference in performance if you're strictly looking at the routers. My guess would be antenna configuration would be the big factor in performance (and as you've probably noticed, the antenna config is about the same on most of the WAPs out there). Something to look at is your wireless network card's performance. How much power is available to the antenna? Can it accept an external antenna?
Another thing to think about is the WAP's physical placement. If you put it in a corner in your basement, you won't have the best performance. If you have a small house, you have nothing to worry about. If you have a large house, then you'll want to place the WAP in the center of your computing activity.