I had about 40% on a WRT54G going through a wall/floor at my former home. I
built a couple reflectors for it out of cardboard, toothpicks, glue and
aluminum foil and got about 10% improvement. Let me know if you'd like the
web site where I got the plans (I'd have to hunt for it). You need a fairly
precise arc for it to work properly/well.

I tried it... and it didn't work well... I think part of the problem is the most direct route from the "away" PC to the router is through the PC's own metal case :/