For RAM: Run the scanner here. click here
Overclocking the processor wont give you the extra speed I suspect you want, you wont get any appreciable increase.
Grahics cards aren't my thing, our gamers will advise you better on that score.
If gaming is your thing and you don't want to spend a lot on upgrading your present computer, new motheerboard, processor, faster memory, faster graphics card, etc. why not think about one of those play station thingies, XBox or something.