At the moment both me and my brother have a low end AMD Radeon HD 6450, but I've just ordered the
AMD Radeon HD 7850 which seems to be the best card in my budget range according to this website.
Firstly, can the left over card be used in combination with either the other 6450 or the 7850 (we both have enough room and pci slots left).
Secondly, will it increase performance and which combination would see the biggest performance boost?
You can crossfire it with the other hd6450 but not with the 7850. To crossfire, they must both be the same series. That said, if you were to crossfire, say, a 7850 with a 7870, the 7870 would be downclocked to match the 7850, so for best gains, it's better to match the same model cards. As for the biggest performance, the single hd7850 would be the one. Also, check to see your psu is powerful enough to run the crossfire set up.