Please remember that any mention of competitors, hinting at competitors or offering to provide details of competitors will result in an account suspension. The full rules can be found under the 'Terms and Rules' link in the bottom right corner of your screen. Just don't mention competitors in any way, shape or form and you'll be OK.
£50 pounds tops
Have you thought about adding whatever you may get from selling the GTS or is that not an option?
Also, why do you want to change? £80 won't get you much further than you are. But double it to £160 and you're within reach of the new 460s which will be a big jump from the GTS.