Currently I've the Logitech X-140 which is connected to the PC used for playing 320KBPS music, I was looking into either a subwoofer or a 5.1 speaker which is used for playing 320KBPS music, gaming and movies on TV, which should I go for?
I wouldn't try adding a subwoofer to the X-140's, it would be tricky to accomplish and ultimately a waste of time and money (even assuming you can buy a proper sub woofer for that price). And £45 for a 5.1 system is spreading your budget too far - it won't buy a 5.1 system that's worth listening to.
Why not look at a range of cheap, powered speakers (i.e. speakers that include amplifiers) that are building a good reputation - Microlab (I should be on commission):
This speaker has two analogue inputs switched by a remote control which might be convenient for your situation.
I should add that I've not actaully heard these speakers - I'm going on what I've read over the past few months.