many many issues here.
Your motherboard may have limitations as to what processor can be installed.
A I5-870 mentioned earlier is a quad core with a clock of 2.8.
It has a passmark rating of 5066 and a single thread rating of 1225. A downgrade for gaming but an upgrade for multithreaded apps.
My take... a wash.
If your new motherboard chipset is similar to your old motherboard, there is a fair chance that you will be able to boot into windows using your old "C" drive. If you can, then you need only to install the motherboard drivers that come with your new motherboard cd.
If the motherboards are too different , like going from amd to intel, then you will not usually boot because your old windows does not have the correct drivers. I fear this is the case skipping several generations of product.
Here is a procedure that might work, but I have not tried it:
http://www.dowdandassociates.com/blog/content/howto-repair-windows-7-install-after-replacing-motherboard/
If you need to do a clean install, it would be an opportune time to upgrade to a ssd.
To prepare, use windows easy transfer to export your settings.
Disconnect your hard drives so windows does not try to put a hidden recovery partition on them.
After the clean install, you can reattach your hard drives and import your old settings.
Since you have a new clean registry, your programs will need to be reinstalled.
If your windows is OEM, you technically need a new license. But, MS is surprisingly lenient about letting you reactivate with a new motherboard.
If telephone reactivation is needed, the key question answer is to say that hhis copy of windows is used nowhere else.
Here is another procedure that I have not tried either:
if you want to just change the windows OS installed hard drive you can use this tool located in your windows : C:\Windows\System32\Sysprep
what this tool does is.. it removes all drivers, app licenses and it prepares your system as a new installation that all drivers are removed ( not assigned anymore ) this is what we use in our company basis to capture images and install on many PCs without no issues...
what you have to do it select OOBE > tick Generalize > shutdown option as "shutdow" when it the its done your PC will shut down and then remove the HDD install it in what ever PC you want. it will have all your files, other programs and everything. but not your licenses will be removed for your license security
They mainly want to be assured that you are not a pirate and that this copy of windows is not used anywhere else.
My strong suggestion is to buy a Samsung evo of at least 240gb. It will likely hold the used portion of your 300 gb hard drive.
500gb will be better, ssd prices are down.
Use the free Samsung ssd migration app to move your C drive to the ssd.
You could just do this upgrade and see how you do.
But with a new motherboard, cpu and ram in hand, use the newly copied ssd to try to get windows working on your new build.
That way, all your old apps and files will be untouched in case everything does not work and you need to do a clean install.
As to a power supply, you will need about a 500w psu for a RX-480.
Buy only a tier 1 or 2 unit from a list such as this:
http://www.tomshardware.com/forum/id-2547993/psu-tier-list.html
Seasonic S12II is a very good unit and reasonably priced,
This 650w unit is on sale for $49:
https://www.newegg.com/Product/Product.aspx?Item=N82E16817151096
Seasonic is