New to Overclocking

Hello and I must say I've been a long time lurker here on the forums. Finally have my first STABLE overclock on any PC I've ever owned, lol. Kinda new to OC'ing, have tried it in the past but haven't had much luck until recently. I haven't been into PC's that much in the last 5 or 6 years or so and I just got back into it. I came across a used surplus PC a few weeks ago and bought a new MoBo for it. I'm planning on upgrading my CPU sometime in the future and I'm wondering what I should run for, a good OC'er without breaking the bank. Now my finances are strapped for the time being and any suggestions would be greatly appreciated. I'm looking to spend about 40-50 bucks at the most on a CPU but as it stands now the performance I get from my dinosaur P4 is actually decent. Here's my setup as follows, mostly second hand parts.

CPU: Intel Pentium 4 520 2.8/800/1M LGA 775 @ 3.2 GHz
Cooler: Arctic Cooling Freezer 7 Pro, Copper/Aluminum HSF
RAM: Kingston DDR2 800 1 GB x 2 Dual Channel Kit
Motherboard: Gigabyte G31M-ES2L Rev 2.3 (Video on board, G31, GMA 3100)
HDD: Seagate Barracuda 160 GB Sata II
PSU: Antec Earthwatts 430 Watt, 20+4 Pin
Going from the stock Intel HSF to my F7 Pro my temps went down from 55c under full load to 38c and that is a MAJOR improvement IMO. I was hesitant about using an aftermarket HSF but I snatched this one up at PC Recycle for 15 bucks and I couldn't pass it up. I'm tempted to bump it up a little more but I can't find any info on my ram and I'm not sure if it could handle it. I've seen others with my CPU running it @ 3.8 stable on air but I'm still hesitant. Well anyways top notch forums here and thanks for reading!

http://www.newegg.com/Product/Product.aspx?Item=N82E16819116264

Is the best you can do for the money... Just make sure your board supports it (think it does but go to mfg website to confirm).

Thank you for the reply. I'm seriously considering that purchase although I found an E5200 on eBay for about 35 used. I'm torn between the two! I know if I go used I'll have no guarantee with it. But the price difference is too much to ignore. But in the mean time I managed to get a stable OC of 3.36 GHz on my CPU. Ran the OCCT Linpack test for about an hour with no problems. It bumped my WEI on Windows 7 up a few points (Not that it matters) but overall it's a good OC. My frame rates shot up from 14-20 on CS:S to 20-40 on Dust 2 @ 1024x768 with the game's recommended settings for my setup and that's on the integrated GMA 3100 . So all in all it's playable, most of the time.