Before buying my Airport Extreme I also considered to buy the Apple Time Capsule which is an Airport Extreme with one hard drive and one hard drive only. I ended up buying an Airport Extreme and a Synology DS211+ with two Western Digital 2 TB green instead. I am glad that I did not bought the Time Capsule because in my opinion it is only a router with a build in backup device.
The Time Capsule does not have the many features that an ordinary intelligent NAS has. You can not run services like FTP, HTTP (web server), use RAID and use surveillance.
I am a big fan of RAID and the lack of RAID configuration on the Time Capsule (There is only one disk) is extremely bad and this is intolerable for me. I want backup of my data with a RAID 1 configuration.
The Time Capsule can provide you with data back up but do not expect it to be the best solution in the wold. Go for a NAS with RAID.
RND2110 READYNAS DUO NAS from NETGEAR is a cheap 2 bay NAS with some good features. Cheap in this case is less than 200 US dollars for a NAS with 1 TB storage.
The RND2110 is in my opinion a good beginner NAS for home users. For business I definitely recommend a Synology NAS or a QNAP NAS that will provide more features for business use.
When you buy the RND2110 you will get 1 TB hard drive but it is possible to add another hard drive and I do recommend a NAS with at least two hard drives in order to use a RAID 1 so disk failures is handled properly.
What I do like about the RND2110 is the rock solid NAS chassis like the QNAP NAS devices.
The power consumption is bad. 60 W is way to much for a 2 bay NAS.
My LAN and WLAN is always turned on so my Apple Airport Extreme is running 24/7 and it is a very good at that. The WLAN covers most of my house but there are some places where the signal is low so I have often needed some sort of a wireless network repeater.
I am not a big fan of WLAN because a WLAN is less secure than a LAN. But the WLAN is extremely nice to have and when you can sit in the garden with your MacBook and access your NAS it makes sense.
The Apple Airport Express device is a easy way to extend the current WLAN and even provide a new feature: AirPlay. With AirPlay you can connect to iTunes on your MacBook and play music anywhere in the house that has some sort music device.
I find Airport Express very useful as the device can be placed almost anywhere it just needs some power in order to function. The Airport Express can not act as a repeater unless the current network is an Airport network. I was a bit annoyed when I heard that.
I definitely recommend the Apple Airport Express if you want to extend your current WLAN.