If your PSU has a PCI-E 6 pin power connector, then the best you can get is the HD 7770 as that also uses little power (much less than even the HD 6770). If you do not have a PCI-E 6 pin power connector, then you will be limited to the HD 7750.
The HD 7770 does cost a bit more but is faster than the 6770 and uses less power and runs cooler. If you want the best possible with your current PSU, then the HD 7770 would be the one to go for.
This site has em going for 95 Euro for the HD 7770.http://skinflint.co.uk/eu/775765