PCI video/audio capture cards for xbox 360 [Help]

I would like to start off by saying that i am a but of a newbie when it comes to things like this, so im sorry if this all might sound a little dumb.

So, i am playing my xbox 360 on my computer moniter and they are connected up using a VGA cable.
I am also using my moniter for my computer aswell, these 2 are connected up using a DVI cable. i can switch between my xbox 360 and my computer digitally on my moniter.
I was wondering if i would be able to capture the video and audio being outputted by my 360 using a PCI video/audio capture card that i can put in my computer.
As far as i am aware this is possible, but i just cant find out much details about it.
And in the sound department things are quite abnormal for me. I hope this "detailed" picture can describe to you what is going on.
(the only wires shown are the ones that i think have something to do with the audio)
(If you don't really understand the image, just say and ill try and explain it as best as i can.)
http://tinypic.com/r/2zpv33a/4
With that setup, i can hear both the audio outputs from my PC and my xbox 360 (at the same time) without any changing of wires etc.

So basically what i would like to know is, if i was to buy a PCI video/audio capture card and put it in my PC, would i be able to capture the sound and the video being outputted from my 360 by using the approprate software, and would i still be able to play my xbox 360 with the same picture quality as i am now. (im on 1920x1080 resolution)

Also, would i be able to perchase a good quality PCI capture card for £30?

If there is anything that i havent told you guys that i need to then please tell me, thanks