|Unless your "1.8 dual core" is a Celeron Dual-Core, it's a little faster than the 3.2GHz Pentium D that you replaced it with!|
The Pentium D is based on the old P4 "Netburst" technology. It runs hot, eats power for brekky, and doesn't accomplish much per clock cycle.
Your 1.8GHz chip, whether it's a Pentium Dual-Core or a Core 2 Duo, is based on newer, far more efficient Core 2 technology. It doesn't produce as much heat, merely sips electricity, and does a lot more work per clock cycle. General rule of thumb is that, in order to match the processing power of a Core 2-based chip with a Netburst CPU, you'd have to double the clock speed. You'd need a 3.6GHz Netburst to match the number crunching power of a 1.8GHz Core 2.
Send that Pentium D back to where it came from and plug the 1.8GHz Pentium Dual-Core/Core 2 Duo back in. Or upgrade to a 2.66+GHz Core 2 Duo if you need a little more oomph. Either way, you'll thank yourself when the power bill comes in. :)
Dual-core Opteron 185 @ 3.2GHz
2x 8800GTS in SLI
4GB CL2 PC3200
X-Fi Ti Pro PCI-E
Samsung 24" LCD