4800 is the fastest and capable of getting the highest OC's, BUT remember that it is diminishing returns on your money.
For bang for the buck, I'd recommend an X2 4400. They have 1mb of cache per core like the 4800, and can OC pretty high. People are getting 2.6 - 2.8 with them from what I've been seeing. They are maybe 100mhz behind the X2 4800, and the X2 3800 seems to get up to about 100mhz behind that.