I found this in a Wiki entry on Tomato, a firmware alternative similar to DD-WRT:
"B/G Mode: This may be Mixed (B+G), B-Only (restricted to 802.11b), or G-Only (restricted to 802.11g). If you set this to B-Only or G-Only, connection attempts from the other protocol may be seen as interference. Recommend leaving this set to "Mixed"."
I've read somewhere that using both B and G in your LAN will slow things down a lot, or at least slow the G devices down a lot. So I just assumed that setting the router to G-only would be best. But this quote suggests that's not the case.
I've also read somewhere that if there are a number of nearby networks, it may be better to be on the same channel with them as opposed to being on a different channel where they will be seen as interference. Apparently interference slows things down more than recognizable competing packets. Is the router setting issue the same kind of thing?
If it matters, my router is a Buffalo WHR-HP-G54, and the only wireless device on my LAN is a laptop using G.