If you can see 20 networks, crowding could be a real factor. iStumbler is a Mac app which does a good job of displaying the networks in your vicinity, their relative signal strength, and the channels on which they operate. It also builds a histogram of your connected signal strength, useful for testing as you walk around, duck behind walls, turn on the microwave, etc.
2.4 GHz wifi, the most common flavor, is split up into 11 channels, though all but 1, 6, and 11 overlap to some extent. If there's a nearby network on an adjacent channel, it'll conflict more with your network than one several channels away. Most access points are set to automatic channel selection, so they'll hop around trying to find an open chunk of bandwidth, which with enough networks quickly becomes fruitless.
The Time Capsule can optionally broadcast using 5 GHz 802.11n, which is both higher performance and less crowded. If your computers are fairly recent, they can join a 5 GHz network, probably at much greater distances than a 2.4 GHz network.
Unfortunately, very few phones and gadgets support 5 GHz wifi, and no iPhones/iPods do. (The iPhone 4 now supports 802.11n, but only in the 2.4 GHz range — very annoying. The iPad does support 5GHz.) So even if you can use 5 GHz for your laptop, it's not a complete solution.
If your Time Capsule is newish, it's capable of dual-band operation, meaning it can pump out a 5GHz network for compatible computers, and a 2.4 GHz network for devices that can't. Unfortunately if you had one of these, and a compatible laptop, you'd probably be using 5GHz already, without knowing it.
I used to have an older, single-band Time Capsule, which I set to 5 GHz-only, and an older wifi base station which I connected to the Time Capsule and ran at 2.4 GHz, giving me the best of both worlds. I didn't have as many networks to contend with as you do, though.
If you're on good terms with your neighbors, maybe you could get together and split a Speakeasy ADSL2 account, consolidating multiple access points and clearing the air. In the old days at least, Speakeasy had an enlightened attitude about sharing connectivity.