Yeah the 6670 looks like a good deal. It's even more powerful and more power efficient than the previous 5670 so there shouldn't be any risk to the system in installing that card. It also overclocks like a charm..
If the PSU is true to its specs, it should be able to run a 5770/6770 with one of those PCIe adapters. The 6670 isn't so bad but I definitely wouldn't call it "high end".
