The 4890 is significantly faster. You won't get some of those nice DX 11 effects, but then the 5670 is too slow to play games in DX11 with decent quality
. The only concern is your PSU as the 4890 can use up to 190W on it's own and those cards generally need two PCI-E connectors.
If you have a good PSU then go with the 4890. If you have a weak PSU go with the 5670.