Internet to the garage?

The first thing about 802.11N is that while it may be newer, there is one major difference between your old router and a newer one with wireless N. 802.11b has a good distance that it can have, however the through put is lower ( ie like you said only about 2mbps). While Wireless N has a higher through put, it has a lesser distance that it can support. However, in your case I do not think that will be a problem as you will have a wnhde111 at the other end. However, you also have to factor in if there are storms, that you may have a slightly lower signal. I would try to have both the WNDR3700 and the wnhde111 as close as possible though. If you upgrade to wireless N, you not only have to take into account what you are trying to do, but also if your devices also support Wireless N. However, if you have the means, I would go with an underground line out to the garage. I would also use a steel conduit over a PVC one as it will last longer and underground varmints wont be able to chew through it and cut your cables.