The HD5770 is certainly a much better card. We need more info though like what resolution you will be using and the power supply in your system. Does the motherboard allow for overclocking by any chance?
Here are some benchmarks for WoW with current cards;http://www.techpowerup.com/reviews/AMD/HD_6670/17.html
The HD4670 isn't included but it is slightly faster than the HD5570, by about 10% or so. Keep in mind those numbers are with a highly OCed i7. If i recall properly WoW doesn't really use more than 2 cores and with the low 2.5ghz base speed of your CPU it might be the limiting factor depending the settings/resolution. I believe large raids like you are talking about are particularly hard on the CPU which is why I ask about the overclocking possibility.