http://www.cpubenchmark.net it gives you the list of all the CPUs out there, even the really old ones.
Let me do a little table for you:
-Price-|-----------CPU------------------------|---Benchmark score
$67.99-AMD Athlon II X2 255 3.1GHz------------|--1832
$74.99-AMD Athlon II X3 440 3.0GHz------------|--2553
$79.99-Intel Pentium Dual-Core E6500 2.93GHz--|--2055
$99.99-AMD Athlon II X4 635 2.9GHz------------|--3375
So it looks like:
The Dual-Core E6500 2.93GHz is
13% faster than the Athlon II X2 255 3.1GHz
The Athlon II X3 440 3.0GHz is
40% faster than the Athlon II X2 255 3.1GHz
The Athlon II X4 635 2.9GHz is
85% faster than the Athlon II X2 255 3.1GHz
Lets eliminate the AMD Athlon II X2 255 3.1GHz because for $7 more you can get the AMD Athlon II X3 440 3.0GHz and that cpu has 3Cores and also
40% faster than that 255.
Now lets eliminate the Pentium Dual-Core E6500 2.93GHz because it's more expensive than the AMD Athlon II X3 440 3.0GHz and also has a lower score than the powerful AMD Athlon II X3 440 3.0GHz. The Dual-Core E6500 2.93GHz is
25% slower than the AMD Athlon II X3 440 3.0GHz and more expensive, so bye bye E6500!
So that comes down to the AMD Athlon II X3 440 3.0GHz and the AMD Athlon II X4 635 2.9GHz. The quadcore Athlon II X4 635 2.9GHz is
33% faster than the AMD Athlon II X3 440 3.0GHz.
So you can buy the powerful AMD Athlon II X3 440 3.0GHz for $75
Or for an extra $25, you can get the AMD Athlon II X4 635 2.9GHz which is
33% faster.
I would go for the AMD Athlon II X4 635 2.9GHz for the extra $25