Upgrading old pc for gaming purposes- graphics card and processor

I have a dated graphic card, a Geforce Series 4, on board of a not very recent pc-config. I have a dual-core processor and only 2 gigs of RAM. I have a limited budget and I would like to increase the performance of my PC for gaming purposes. Firstly, can I know how to overclock both my processor and graphics card? Secondly what is the cheapest upgrade to drastically increase the processing speed?
