[Advice Pro]

The single core CPU that I can think of are:

AMD Athlon
AMD Athlon 64
AMD Athlon X2
AMD Athlon XP
AMD Duron
AMD Geode
AMD Opteron
AMD Sempron
AMD Turion 64
Intel Celeron
Intel Celeron D
Intel Celeron M
Intel Core Solo
Intel Itanium
Intel Pentium
Intel Pentium 4
Intel Pentium 4 Extreme Ed.
Intel Pentium 4 HT
Intel Pentium D
Intel Pentium III E
Intel Pentium M, Centrino
Intel Pentium MMX
Intel Xeon
Intel Xeon MP

[Kernel_Killer]

You posted a few dual and quad cores, but it depends on what you plan to do with the system.

Geodes are great single cores for embedded light-weight systems, while the something like an Athlon64 would be a great system-based single core (unfortunately, they are hard to get, and the prices are rising for replacements). The second best single core for a system would be P4 Northwood.

Practically speaking, if you are wanting to go with an actual system to use as a desktop or server, I'd advise a AMD X4 processor. You would spend as much as a Athlon64 (around $80 at least), and a good montherboard (like a DFI), for about $70. The DDR2 RAM it takes would run about $50 for 2x1GB sticks.