@Cerunnir
I bought a 6870 nearly 2 years ago for around £120. The cheapest I could have paid for a 7870XT was £172. Thats a total of £292.
If I had splashed out on a GTX 580 when they had first come out, £400-£500 easily. The GTX 580 on average is around 5-10% weaker than the 7870XT and hass less vRAM.
Building a high powered system is NEVER cost effective. If price wasn't an issue, the OP wouldn't be on this forum asking for advice on the best 600s. He'd simply have bought the best one already.
Edit
Also I think the 680 would struggle to fit into an HP chassis. You should check the dimensions and more importantly the PSU as someone has mentioned earlier.