I have the same question. Anyone? I know it is a dodgy issue and I shouldn't be doing it unless I'm sure I know what I'm doing, but still, I need to start learning somewhere! Please help!
If you have a P35 chipset it is very easy to overclock.
By the way, how much you can overclock is dependent upon several factors. You need a high quality power supply to provide clean power.
For an old CPU, as the original poster, you should have a motherboard with 6 phase for CPU for a 2 core CPU and 8 phase for a 4 core. [Look up motherboard CPU phase on the internet.] Power to the CPU goes through each "phase", therefore, the more your motherboard has the cooler they are because power is divided between each one.
Need RAM that can stand an overclocked CPU. And, much better CPU cooler than comes with a boxed or retail CPU, make sure you find the measurements on the one you want to purchase - to be sure it will fit your case.
Start the overclock slowly, you will have to increase voltages at some point, so, increase them a little at a time. Most likely the CPU will need more attention for bumping up voltage as your overclock gets higher.
With an unlocked CPU, as the OP, all you have to do is enter BIOS [look up the proceedure in your motherboard manual, or sometimes it is on your splash screen if you have a custom built computer], and begin increasing the CPU multiplier [again this is on a P35 motherboard, later chipsets were much more difficult].
There are lots of places to find overclock info, I think tomshardware.com and anandtech.com even has articles.
Beginning with P45 motherboards the overclocking process gets much more difficult until you get to Ivy Bridge, sorry, I cannot remember the difficulty level of Sandy Bridge overclock, I have a memory that often fails me.
Sandy Bridge is about the same ease to overclock as a P35 chipset motherboard, except you must use a great deal less voltage increase. I did a very quick slight overclock on my i5 3570 to 4GHz and the CPU is at 1.115 volts. I am using a modest CPU cooler, Noctua NH-C14 140mm x 2 SSO CPU Cooler in a HAF 922 case [3x 200mm fans + 1 each 120mm at the rear].