The difference between multiple processors and multiple cores

My application works with 4 simultaneous threads. I can make these threads both multithreaded and single-threaded (MTA / STA). When I declare their STA, the result is faster than when I declare their MTA. Its written in vb.net, and I use the standard version of server 2003 (Windows) as the operating system. The processor is single-core and quad-core.

What to do to make it faster?

I can increase the number of processors to 2, or use a hexcore / octal processor instead. Can anyone suggest?

Thank.