Could someone tell me my bottleneck and what I can do about it?

Mobo: MSI K9N2 SLI Platinum/ K9N2 Zilent   (MSI-7374)

CPU: AMD Athlon 64 X2 6400+

Graphics: Nvidia Geforce GTX 580

Memory: DDR2 8 gigs


I think it's my CPU... What's the best CPU I can put in there? I saw the list on MSI but I've heard people putting even better CPU's in this board than the list says but needing to do something with BIOS.

 This is for gaming by the way.

Would it make sense to build a completely new PC?

you are correct, it is your cpu. because it is an AM2 socket, to upgrade, you would need a new mobo, new cpu, and new RAM.

Well on the site it says I have AM2+ and that it's AM3 ready. I'm trying to avoid building a new PC if I can get substantial performance out of this one.

it depends on how much money can you afford...

You can go up to a Phenom X4 or x6 on that mobo but it will probably be better just to start bilding a new pc 

an Phenom iiX4 955 will be compatable

what kind of cost am I looking at to build a new rig though? How much performance would I get out of upgrading to the best possible CPU this mobo can have?

If  you swaped out that Athlon 64 X2 6400 with a Phenom iiX4 955 you will have more than 3times the power and 4 cores.

Its still a good chip 

building a new computer can get expencive