Good, if you have money to spend things get more interesting .
I can only imagine it would be like: Guitar > Boss Pedal > Audio Interface > PC > PC Speaker
This is nearly correct. There are a few scenarios, each have different routings depending on the options your audio interface will offer. Generally speaking, everything sound related will connect to the audio interface.
When working with Cubase the signal flow will be:
Guitar > Pedal > Audio Interface > Cubase > Audio Interface > Speakers.
When working with the PC but not Cubase (IE listening to music or videos) the signal flow will be
PC > Audio Interface >Speakers.
Lastly, some audio interfaces offer standalone operation, so you could, with your PC turned off, use
Guitar > Boss Pedal > Audio interface > Speakers.
(Whether you want that is up to you, if you have a good guitar amp for instance I doubt you’ll want to use your Speakers in this case. I see the pedal also has a headphone output so for using headphones you can bypass the audio interface completely if you want to.)
This does however mean that you need to have speakers that you can connect to another soundsource than your PC. With built-in speakers like in a laptop this won’t work.
As for which interface to get: plenty of choice. USB or Firewire doesn’t really matter, although firewire performance is VERY dependant on which firewire chip you have. It is probably safer to go with USB, also because there’s more on offer and most cheaper interfaces are on USB anyway.
Now you will need at least 2 inputs (the pedal offers stereo output), and of course 2 outputs for your speakers. Most PC speakers connect with a regular stereo minijack, a connection most audio interfaces don’t offer. That means you’ll need to either get a converter cable from stereo minijack to 2x mono jack, or use the headphone output on the interface with a common minijack-jack converter plug. Audio interfaces don’t offer minijack connections because higher end speaker systems support balanced connections, in which case dual mono outputs are required.
What to get: as said, there is plenty on offer. I don’t know what computer you have, but Windows/Mac is obviously important, as you need compatible drivers. I see you have W7, do you run a 64 or 32-bit?
Steinberg offer their CI-range of USB audio interfaces which fall into your budget. Lexiconn, M-Audio, E-MU, Line-6 (these are especially geared towards guitar users, but you have that covered with your pedal already.), Tascam, ESI, Focusrite, Presonus, Mackie etc etc.
I obviously don’t have experience with all of them, but I’ve heard mostly good stories about most of these, apart from ESI who I have never heard anyone mention.
I could give you specific models but I suggest you just see what’s available in your area and closely compare the different options. Read some reviews, ask specific questions here if anything is unclear. In most currencies 300,- can buy you a fine audio interface