The pentium 4 is a downgrade from the Pentium D your machine came with. If possible, can you list which Pentium D you have? I have a Pentium D 820 and Pentium D 915, both of them are 2.8GHz clock but different cache size. Yours may be totally different but, can you tell us which yours is? If you need help finding that information, download CPUz (the non-installer, .zip file) and list the "Name) under the "Processor" heading. (Directly next to the picture :-))
CPUID - System & hardware benchmark, monitoring, reporting http://puttsy.me/qDmttX
Let me know!