You have one of the faster p4's made. If your board isn't listed on the hp website with supported cpus, then I would contact hp about an upgrade. Your board bios will determine what cpu it will recognize. I used compaq's free online chat to discuss options with my compaq pc, and ended up changing out the motherboard and reinstalling windows using a dell windows dvd. The old p4's above 3.0 ran pretty warm. Fry's had an e5200 with board for only $80 last week that would have fit in your hp case. You'll have to decide for yourself if you're capable of changing out a motherboard and are willing to format the hardrive and start over, or back up your files first before formatting.
Thank you all guys ! I am not thinking of changing the inside of my HP SFF, everything is in order and I am sure no mobo with fit 100% in this case without replacing any other part. for example the CPU is in front of the case (near the air grails) and not in the middle as we can see in many mobos. Just please let me know exact what is the type of the fastest CPU I can upgrade, for example P4 650 ? E5200 ? or something else ? when you inform me about that I will login to ebay and buy it , just like that !!!
Thanks again in advance everyone of you, you are very kind !