I believe the 7770 would be as high as you can go just to be safe, but you can always go for a gpu that is a little better and fit in a 7790 without the worry of bottle neck at all. But for your case I would suggest the 7750 or the 7770, but not the 7790 just to be safe. Hope this helped!
I had an athlon x3 at 2.5 GHz and it massively bottlenecked the 7770 I had with it. I would buy the 7750 if I were u unless there was a very small price diffence
The x3 440 was able to handle the 5870. Therfore the x3 450 can handle the 7750 or 7770 ghz edition. I read up on it and thos has been tested. I dont know why it bottlenecked for you, but it should not and also if you were to overclock your x3 450 it would help quite a lot.
I didn't have a 450. I had a triple core apu which uses the athlon II architecture and I had over clocked it to 2.5ghz. I bought an i3 and got like 15fps more
Then do not compare it to this he has an athlon x3 450 that is clocked at 3.2ghz which is better than the one you had. He wont have problems especially if he overclocks.
Its just clocked different... Yeah it will bottleneck less but since I was bottlenecked SO much he will still be bottlenecked with a 7770. Like I said I'd still buy it since normally they run $10 more than a 7750 just it will be bottlenecked some