C2D = Core 2 duo
C2Q = Core 2 quad
Intel's C2D beats AMD Athlon X2's and beats there Phenom X3's
AMD's Phenom II X3's beat Intels C2D but does not beat Intels C2Q
AMD's Phenom X4's are horrible quad cores, stay away from those.
Intel's C2Q are about equal to AMD's Phenom II X4's
Intel's Core i7 beats any AMD CPU but it is also the most expensive CPU. The i7 CPU uses Tri-channel DDR3 memory and currently the only available motherboard you can use with them are X58 motherboards which are quite expensive.
C2Q and C2D use LGA 775 as a socket (Intel)
Core i7 uses LGA 1366 as a socket (Intel)
AMD's Athlon X2's use Socket AM2 And AM2+ (both compatible with each other)
AMD's Phenom X3's and Phenom X4's use socket AM2+ but will work in socket AM2
AMD's Phenom II X4 uses socket AM2+ but will work in socket AM2
AMD's Phenom II X3 uses socket AM3 but will work in socket AM2+ and AM2.
If you are looking into overclocking Intel's 45NM CPU's overclock like mad, and AMD Phenom II's are also good overclockers.
IMO Intel beats out AMD for quality, but AMD has the best bang/Buck ratio.
If you do decided to go AMD it would be a good idea to stay away from the Phenom I CPU's.