Some doubts about my cpu build

Hello guys

My situation is this:
Im trying to build a new computer, as my actual one is giving me some trouble, and i think it already did it's time (3 years). I have an AMD Athlon 64x2 Dual Core 4200 (2 x 2.21), 2 gb ram.. 320 gb disc, Graphic card ATI Radeon X1950 Pro..

Well, I'm thinking of getting an Intel i5-750 8M Cache, 2.66 GHz or an Amd Phenom Ii X6 1100 T 3.3ghz 9mb Am3, i've not really decided between these 2 yet, so i would like your advice
I would also like to know wich motherboard could be good for them, I had in mind one of these 2:
Asus M5a97 Evo Chipset 970 Am3 or Asus M5a88-m Amd 880, do you think these are any good for the micros?

I'll get 2x4gb dd3 ram (8 gb), a 2tb hard disc, and the most important thing for me, the graphic card, I was thinking of a Sapphire Radeon 6780 1gb 256 bit ddr 5, but i would love to hear advices on this one too..

The use that I'll give to the pc is mostly gaming, a lot of gaming, audio editon and recording too, and probably lots of multitasking...

I'll appreciate your advise about that build, wich ones are good with each other, wich ones aren't, and every other thing you would like to say.

Thanks a lot!