It may also be a network problem.
You said you connected the Time Capsule to an existing network: what kind of network?
To use WiFi "n", your wireless access point (generally, your router) has to support WiFi "n" too.
Also, even when using WiFi "n", depending on your router (and its antennas) you may get different speeds, from 300 mbps to 450 mbps.
Last thing about the WiFi connection: the channel you are using may be "congested". This is especially possible if your neighbors have WiFi networks too. You may want to try with a different channel (you can change that in your router's settings panel).
Did you connect your Mac to the Time Capsule directly? If the Time Capsule is connected to a network, you should never connect to it directly (it won't work as you expect!).
That may also explain why you are not seeing differences in speed: your Mac was still using WiFi to connect to the Time Capsule.
The best thing to do, in my opinion, is to connect the Time Capsule to the network via gigabit ethernet.
First of all:
- Make sure you have a router with gigabit ethernet ports (or it will be actually slower than WiFi "n"!)
- Make sure your cables can support gigabit ethernet (have to be "class 5e" or higher, preferably)
Then, connect the Time Capsule to the router via cable. You may then connect to it using either your existing WiFi network or via gigabit ethernet (plug the cable into the router, not into the Time Capsule!).