just stick to either the hd6670 or hd6570. Your 255w psu won't hold anything higher. Both of those cards have dual link dvi connectors.
Actually, it looks like 16amp is a bit too low for the hd6670, just get the hd6570. Getting the hd6670 would be too risky since your psu is well below 300w.
Sorry, scratch the bit about the 6850, I forgot the PSU was just 255W. I looked at some 6570s and none had dual-link dvi, just a dvi and vga. I suppose it will work fine with an adapter, so go for the 6570.