First of all, just because a particular processor is said to be compatible with the 945P chipset on your motherboard does not mean that you can use this processor with your motherboard. It is your motherboard's design and its BIOS that will determine whether or not a particular processor will work:
- The voltage regulators on your motherboard must generate - without overheating - enough power for the processor that you install. I would note that none of the Intel-produced 945P motherboards support the Core 2 Extreme processors. If your motherboard is from another manufacturer, you should check their documentation for information on the processors supported.
- The BIOS on the motherboard must include support for the particular processor that you use. If you attempt to use a processor that is unknown to the BIOS, things likely will not work properly.
If both cases, for us to provide you with any better information, we need to know more about the particular motherboard that you have. Please provide the manufacturer and model number for your board.
Now, to your questions. The maximum amount of memory that this 945P chipset will support is 4GB. This can be accomplished using two 2GB DIMMs or four 1GB DIMMs (if you motherboard has four DIMM sockets). You will need to consult the specification for your motherboard to determine this and other necessary DIMM specifics.
Finally, a comment. You motherboard is 10 generations out of date and likely 9 years old. This is well beyond the warranted lifetime for this motherboard - and indeed well beyond the MTBF (Mean Time Between Failure) for the components used on the motherboard. My recommendation would be to not waste any money trying to upgrade the processor (and/or memory). Instead, I recommend that you spend your money purchasing a (much more) modern motherboard and processor...
Hope this helps,
Excellent information from Scott.
You could also check the following link for hardware recommendations: http://www.intel.com/content/www/us/en/processor-comparison/how-to-choose-the-right-processor.html?wapkw=help+me+choose