is it dual core or is it not......

A dual core, non-HT CPU will show two processors in task manager.

A single core, HT CPU will show two processors in task manager.

A dual core, HT CPU (Extreme Edition and Xeon chips) will show 4 CPUs in task manager - if the OS supports multi-core and HT CPUs.

You can try to use WCPUID to determine the number of cores and the status of HT:
http://hp.vector.co.jp/authors/VA002374/src/download.html

Or, other programs such as SiSoft Sandra may be able to do it. Lastly, you can try to watch and read the POST screen to see what it says as the computer boots.

I also recommend going to CPUID.com and using the free utility out there. It will give you the exact model number.

Then, I like to keep these CPU charts bookmarked:

(INTEL)
http://images.tomshardware.com/images/tables/cpu_table_intel_big.gif

(AMD)
http://images.tomshardware.com/2005/11/21/the_mother_of_all_cpu_charts_2005/cpu_table_amd_big.gif