Basically, see heading. I'm not too great at picking good components, so I've put off upgrading my CPU In the past hour and a half I think I've learned next to nothing about a workable upgrade.
I really want a new CPU. I'm pretty sure that'll take a new mobo.
-4GB of DDR2 I think I'd rather not change unless It's a good idea to.
-AMD Athlon 64 X2 5200+ (socket AM2)
-Asus M2N-E motherboard, has an AM2 slot (Right?)
I forget my psu but I think it'll handle anything you can throw at it.
The absolute most I'm spending is $300. I'd really like to spend <$200.
Well the best you can put on that board is a Phenom x4 which will cost you around 100 to buy new for the fastest one. I would not recommend doing that. You should upgrade your MoBo and processor to an AM3 socket for about 250 you can do this and reuse the ram that you have now.