IIRC, Core architecture (that is, the architecture sold as the Core 2 line) is a widened Pentium M design, essentially. Quad-issue instead of triple-issue like all P6 (Pentium Pro through Core Solo/Duo) chips, and it does have some improvements in the x86 instruction decoder, it seems.
I believe Nehalem architecture (Core i) is (oversimplified) Core with HyperThreading and higher clock speeds, essentially.