Typically a core clock is not a good way to determine how good a graphics card is. For instance, you can find an HD 7870 GHz clocked at...well...1 GHz. You could then compare it to an HD 7970, clocked at say...925MHz. The 7970 would blow the 7870 GHz out of the water. In terms of bang for buck, the GTX 760 is the best performer, and it is only $250 (~£160)
Typically a core clock is not a good way to determine how good a graphics card is. For instance, you can find an HD 7870 GHz clocked at...well...1 GHz. You could then compare it to an HD 7970, clocked at say...925MHz. The 7970 would blow the 7870 GHz out of the water. In terms of bang for buck, the GTX 760 is the best performer, and it is only $250 (~£160)