Although it's not terribly scientific a measurement, I'm presently copying DVDs from my wife's laptop (*) to my file server. It's taking about 30 minutes to move 5GB, which is about 22 Mbits/sec. I suspect that the bottleneck is actually the DVD drive overheating. Anyway, my Linksys WRT54GS seems to go quite fast. There's about fifteen feet, one wall, and one full bookcase between the computer and the base station.

I've experienced some slowness in the past, and I was always able to solve it by changing channels. I know you've tried that, but you might sequentially try each different channel and see if you observe any difference.

(*) This was the result of paying a commercial ripping service to chug through my CDs and rip to Apple Lossless format. I got 45 DVD+R's back. My Mac's "SuperDrive" is DVD-R, and has weird errors on some DVDs, so I'm using the laptop instead.