did you install all the drivers live the motherboard chipset drivers, it may help if you download the latest drivers for the following from the required sites, like your Chipset Drivers, AGP Card drivers (click here) are the main ones then do the windows updates !
You do not specify the make & Model of you Motherboard, maybe there is a BIOS update needed to support the CPU, large amount of memory and the FSB. This can be done by reading the support from the manufacturer's w.site.
Have your applied the thermal conductive paste to the CPU Core ? I take it This AMD CPU is a AMD Athlon XP3000 if so this is not a 3GHz CPU its a higher core spec on the FSB unles you have gone for a highend CPU in the 64Bit.
What type of Memory ? PC2700 pr PC3200 ? is this matched to you motherboard. Also the settings in the BIOS ? are they correct ?
I perosonally think you have tried too much change too soon and you need to illuminate 1 thing at a time.