What is it exactly that you want to do with the new card ? What games and at what resolution ? one or more monitors ? Also what power supply do you have ?
Im thinking a 5850 may be a better option but its dependant on your usage.
Well i would say that i would go for the 5870 personally. As you dont think you will SLI anyway then the single ATI card with its Eyefinity support would be the way i would go.
Your PSU seems fine so no worries there but the CPU is a little on the weak side for that card.
Here is an interesting article Toms did about CPU's and gaming http://www.tomshardware.co.uk/core-i3-gaming,review-318...
Going from a 9600GT. You will be just as happy with a HD5850 as HD5870, it is a huge leap in graphics power and it is a better match to your CPU (people in here are reporting overclocks up to the level of 5870)