In our case, Wicked Defense (including incoming v1.5 release) and another project we are developing are all single-threaded.

These games aren't CPU extensive, so they can run on P3 1.12 Ghz with an average of 50% CPU usage. The graphics, however, is another question. Do we have multi-core GPUs already (that aren't SLIs)?

Another challange is Direct3D DrawPrimitive overhead, which even with instancing increases CPU usage and reduces the performance, and is not solved on multi-core CPUs. We have yet to see if DX10 reduces this problem at least partially.