The most important specification, that you left out, is the make and model and revision level of the main board everything is attached to, into which you will be inserting the RAM!!!
DANGER is that the "2 old RAM sticks, laying around" will blow up your nice gaming machine, because they aren't in static proof sealed packages, you probably have been handling them without ESD protection.
If you can afford to replace the total batch of equipment, and you wear an anti static strap, then, you might try the sticks one at a time, powering down, to remove and install them.
But, the entire RAM stack runs at the speed of the SLOWEST stick! So, if they are even compatible, would it be worth the effort?
I would test the memory in one of my old machines that I can afford to explode.
Answer to Question #2 - Machine slows down...
Microsoft is of course what you are running, because of the 821 Operating Systems out there, most of which are FREE for the DL at http://livecdlist.com and http://distrowatch.com ,
ONLY Microsoft has the background 'housekeeping' prioritized to interrupt and stall the User processes, in order to 'phone home' and report all your keystrokes, latest download URLs, etc.
No amount of memory will 'fix' that Microsoft chaos. Get used to it in your gaming.