I have a question about getting a computer built.?

It should run Office fine right now, maybe get some more memory and upgrade to Windows 10. you wont see much of a difference in office with a complete overhaul. but it would play things like games better. 

Well you can still do some things with that PC if you were to bring the RAM to 8gb and you replaced the 640gb drive with a 500gb or 1tb SSD.

The q6600 was and still is an iconic processor. It's still capable of web-surfing and moderate office work. However one of the reasons the q6600 was iconic is because the later models could overclock well which you can't do with your setup. Overclocking the q6600 would make it competitive with a 1st Generation Core i5 processor.

If you're trying to crunch huge sets of data with Excel then you should look at a replacement. If the CPU usage is at 100% then your CPU is the limiting factor.

The board is a run of the mill mATX motherboard that's made by Foxconn. You should easily be able to find a replacement for it. Be mindful and do research on the case header that connects the power button and HDD lights to the motherboard. Some companies use a 9-pin header that resembles a USB 2.0 header and some don't. This will make connecting the motherboard to the case less of a hassle.

I would highly recommend replacing the power supply and in fact you may be forced to do so since the current PSU in your PC most likely has a 4-pin CPU connector. Pretty much any modern motherboard uses an 8-pin CPU connector. The 450w Corsair CX450 is a good choice.

i would go to a computer place and ask them all these questions

If you want to keep that tower then the first thing you have to find out is the Form Factor of the Motherboard in that tower.  Form Factor is the Size of the Motherboard.

Then when you know the Form Factor you can pick the right new Motherboard.

Motherboards will accept two brands of Processors (CPU),  AMD and Intel and they are not interchangeable. So another decision is do you want AMD or INTEL?

Lets Say you Pick Intel CPU,  Then you get a Motherboard that will support an INTEL CPU.  Then you pick the CPU you want  i3, i5, or i7   and make sure the CPU Socket matches the CPU Socket on the Motherboard.

Next you purchase RAM for that Motherboard.  Then a Hard Drive. 

Back to the Motherboard. Some Motherboards come with a GPU (Graphics Card) and some don't.   For what you need, just get a Motherboard with a GPU.

Next you will need a Power Supply,  get at least a 800 Watt PS>

Then you will need an Operating System (Windows 10).

As for Office Software, ApacheOpenOffice is free.