I agree to wait but the price is some where around 150 dollars the 7750 is 140 dollars. Now if you go the other way the 7870 looks like 249 dollars the 7850 is about 230 dollars. That is what they are estimating them to be.
The PSU you have is on the tier 3 list it is on the line between good and bad. So before you buy any of the 7xxx series make sure that PSU will drive the new card. Here is a link to the list. The list has been arouund a long time but it is still a good tool to use.
I guess I forgot the link well here it is sorry about that. Thats what happen's when you get old. Also depending what the watts or amps that GPU will require and if the amps on the 12v rail will be enough. You might be OK but again we are talking about money invested. Good luck to ya