sempron is not a dual core processor.

This questions is not right, try with clock memory or AMD Sempron but not both in one !

PowerSpec 5500 system has AMD Sempron processor (Sempron 3100+) running at 1.8GHz (1800MHz)

I believe I read somewhere that the Intel Celeron line was the equivalent to the sempron line

The Sempron line replaced the Duron line beginning in 2004. Sempron processors are faster than Duron processors by the very nature of being newer, though they target the same market segment.