Hello,
I want a gaming computer, i cant afford a new processer but i want max peformance. I have geforce 6600 256 mb but games like doom 3 run slow. i want to overclock my processer but i dont know how. i want to get it around 3.0 ghz. its 2.6 at the moment

Well first of all your computer specs would be nice because then people can help you more.

If your computer is a namebrand pc you miles well forget about it, because you have zero bios options to overclock your system. Using software overclocking programs may get you an extra 100mhz or 200mhz max, but that's it, considering you won't be able to lock the agp/pci bus adjust the vcore and the FSB.

AMD Athlon 64 X2 4600+ OC 2.7ghz
2GB Dual Channel DDR 3200
Nvidia 7900GT
SATA II 2x 200gig 7200rpm 16mb cache RAID-0
Gigabyte Nforce 4 SLI

Your celeron has 100Mhz FSB and 26x multiplier... Simply change the FSB to 133Mhz to see if it works. My friend have a 2.6Ghz and I overclocked it to 3.46 simply switching to 133Mhz FSB by jumper. There is no need to adjust vcore and plus, he is using a ECS motherboard. If the BIOS is locked and you can't find such jumper on the motherboard, even though you using software to overclock, your system won't to stable because AGP/PCI/RAM will all be overclock beyond the spec, making it difficult to increase CPU frequency w/o affecting those. If that is the case, your average overclock will be around 5-15% and won't make a huge difference in terms of performance from a Celeron... 128kb L2 "2-WAY ASSOCIATIVE" cache comparing to P4 512kb L2 "8-WAY ASSOCIATIVE" cache is where the performance hit is... My suggestion for you is buy a cheap used S478 p4 on ebay for cheap and replace with your current celeron and then sell your old one on ebay...

TMP-Man

Asus P5P800-SE
P4 506 @ 4200Mhz 1.45v
Thermaltake CLP0024 w/ 1700RPM 92MM + AS5
1GB Corsair 2-3-3-5 DDR400
120GB/300GB 7200RPM HD
MSI 6600GT AGP 550/1150