And in my view, one that you can keep intact!
First, the "all in one" that you have actually has four functions -- modem (your Internet connection to the world), router (sets up a local area network, LAN, and does Network Address Translation, NAT, between the outside world and your LAN), Ethernet switch to allow multiple hard-wired connections to your LAN, and a WiFi wireless access point. It may also have a USB port for connecting a hard drive as a NAS (Network Attached Storage). Typically any of these functions can be disabled if you don't need them -- the only one you absolutely need is the modem function.
The Time Capsule is a NAS, router, switch, and wireless access point. The only feature it doesn't have is that of the modem. You could physically connect it to you existing all in one and use it solely as a NAS. Or you could turn on its wireless access point (and probably but not required turn off the one in the all in one) to get the faster WiFi. In either of these cases you would run it in "pass through" mode, which disables its router functionality, as you already have that in the all in one. If you only wanted the wireless access point you could halve the cost buy buying the AirportExtreme Base Station.
As I said earlier, since you have only one computer and it is a desktop system, you will get the fastest performance as well as the lowest cost by attaching an external hard drive directly to your computer. You can buy a 4TB external hard drive for less money than even the AirPortExtreme. Also with only a single computer, your WiFi is used only for Internet access, which is slower than your current WiFi, so there would be no speed advantage to the AirportExtreme or TimeCapsule's wireless access port.
