Well, put a few bucks into RAM, and if your computer is OEM, then no OC. If it is OEM, grab a hd 5830. When OCing, grab a decent cooler like a Corsair A70.
http://www.newegg.com/Product/Product.aspx?Item=N82E16814150447&cm_re=hd_5770-_-14-150-447-_-Product
OCing is potentially dangerous, but if you do it right, then you will be golden.
By raising your FSB or BCLK (Front Side Bus and Base Clock) you can overclock. When you raise your FSB, you can fry your RAM, so the first step is to put your RAM down a step. e.g. 800Mhz RAM to 667Mhz, 667Mhz RAM to 533Mhz. Then when you OC, it will bring your RAM near the stock performance. Raise your FSB a few mhz at a time, and then your multiplier will bring your CPU clock up. Your multiplier just multipliers your FSB. So 200Mhz FSB x 15 multiplier = 3Ghz. So, you might need to raise your voltage. Here is the safe voltage: 0.8500V-1.3625V By raising your voltages, you can achieve a more stable overclock. So, after you turn down your RAM and turn up the FSB, you will also want to put up your northbridge a bit. Now you have put your RAM speeds down, FSB up a little, and southbridge up a bit, you run a stress test.
My favorite:
http://www.ocbase.com/perestroika_en/index.php?Download
Run that for four hours with Large Data Test and Priority Normal. If it passes, your clear. Keep adding your CPU FSB Mhz little by little until you start getting crashes when booting. If you can get 3Ghz, your golden. Remember, it can be dangerous, so don't go all crazy. Also download CPU-Z to monitor your CPU settings.
http://www.cpuid.com/softwares/cpu-z.html
Don't run a stress test every time you boot. I would say if you can achieve 2.8Ghz, run a test and then keep going. Watch some youtube videos too. I hope this helped!