Does my CPU use Hyperthreading?

From a BIOS stand point..... for most of the Dell models, if the HT option isn't available in the CPU Information section of the BIOS Setup then HT isn't supported by your processor.  The only exception I can think of is the 8200 model which had a short lived A09 BIOS release which supported HT.  That BIOS version was later replaced with another A09 version that didn't offer HT.  For more detailed information on your specific system then post in the Dimension - General Hardware forum.  Include in your posting your full Dell model number and information on your currently installed processor.  The folks visiting there can best advise you if your processor supports HT in your model.