Depends on what you need it for...
If you want the fastest for your money, then depending on stock, the X2 6000 is still a brilliant cpu - it runs at 3Ghz and is dual core. If you want newer technology then you could go for the Phenom but in your price range the only one you could get is the triple core 8450 which runs at 2.1Ghz but is about $10 more. Lastly, if you want lower power consumption, then you could go for the Athlon X2 BE 2400 which runs at 2.3Ghz but uses loads less power. Don't forget to get decent ram to go with it tho - 2Gb is the minimum I would go for and get decent PC-6400 speeds or higher if you want to overclock.