AMD Athlon 64 x2 6400+ vs. Intel Core 2 Quad Q6600

I am planning on upgrading my current computer. I was wondering which one of these processors I should get:

AMD Athlon 64 X2 6400+ 3.2 GHz

Intel Core 2 Quad Q6600 2.4GHz

I can get the processor, motherboard, and 2GB memory for about $500-$550, so I am just wondering about the CPU I should get. Also, instead of the 6400+, should i get the 6000+ instead?

Thanks!

(How much better will the Intel perform over the AMD?)