802.11n is backward compatible with 802.11g so if you are going to buy one and the added cost of the 802.11n is not an issue, I would purchase it. I recently had to replace my 802.11g router and did it with a 802.11n even though I do not have an machine that has an N card. Everything works great and I can upgrade to N cards later.
802.11n is better, however it is still in development. Most "n" routers are in "draft" stage at this point. I purchased an "n" router as an upgrade to my "g"router and the performance increase is incredible. I also use several UMA mobile phones on my "n" network as well as heavy video streamline. All this the same time and it works great. That's because of the MIMO technology of the "n" routers. Multiple in Multiple out allows for more stable simultaneous connections the more antennas the modem has. I recommend "n" routers to anyone even if their devices do not yet support it.