Your CPU support list:
"The BIOS also issues a beep code (one long tone followed by two short tones) during POST if the video configuration fails (a faulty video card or no card installed) or if an external ROM module does not properly checksum to zero. Table 16 lists the BIOS codes.
Table 16. Beep Codes
3 No memory
Siren Processor overheat (on reboot)" from manual
Since you have not included the CPU information, I have included a CPU support list.
Since you have not included information on your power supply, the memory used (DDR2 667 or 800), or details on whether these other components are tested from another working machine, there is little information to go on.
I personally do not see you first purchasing an older motherboard from a marginal source (untested?) with an undisclosed processor, then you purchase another identical processor (from a reliable source), and now you intend on purchasing another motherboard (the same???), and all without complete testing. This is not the right way of going about this project.When you are done, you probably could have purchased quality brand new modern components from a reliable source and been more positive of your project's results, plus components would have included a warranty.
I just noticed you listed the processor with your info. If you have the E6550 CPU, look at the support list and notice that it is supported only by BIOS version 0326, if you have an older BIOS, you may need to flash yours with the correct update. This however will require you to have another socket 775 processor that is supported by your BIOS version in order to check or update the BIOS.... Possibly another expense.
You have to be a little bit crazy to keep you from going insane.