You cannot utilize 4 or 6 GB RAM unless your processor is 64bit. 32bit processor cannot use more than 3GB of RAM
You could throw in 4 gigs, but it really only utilizes around 3.25, so if that .25 is worth the money its up to you.
That PC appears to be a mini tower. You will need a low profile graphics card, and you also need to find the wattage of your psu. Dont expect too much out of this machine for gaming.