Newer games are going to use dual-core more effectively (some are now and older ones are releasing patches to take advantage of it!).
If you can, definitely go for dual-core. If not, no sweat, go with an Opteron 144 and OC it 400-800mhz or so.
Note: If you go dual-core, stick with the Athlon 64 x2 3800+ and the Athlon 64 x2 4400+. The Opteron 165 is also dual core and a good option.
If you go with a lower end single core, go with an Opteron 144, 146 or 148, as they OC extremely well. Athlon 64's are getting outdated by the Opterons because Opterons tend to have twice as much L2 cache as their Athlon 64 equivalent and OC twice as much.