Help Me?

OK I BOUGHT THIS COMP 5 YEARS AGO. ITS AN HP PAVILION 6645C. I WANT TO UPGRADE NOW. WHEN I FIRST BOUGHT IT CAME WITH COGNAC MOTHERBOARD SOCKET 370, 566MHz CELERON PROCESSOR. IT HAD 64MB OF RAM, AND NO VIDEO CARD. IT HAD A MODEM AND A CD-RW. NOW IT STILL HAS THE SAME PROCESSOR, BUT IT HAS MORE RAM WHICH NOW STANDS AT 384MB, I PUT A NEW NVIDIA 64MB GEFORE MX4000 VIDEO CARD, AND A PCI NIC CARD ADAPTER FOR MY DSL CONNECTION. I NOW WANT TO UPGRADE THE PROCESSOR. I NEED TO KNOW IF I CAN UPGRADE MY COMP WITH A NEW 1GHz CELERON PROCESSOR OR PENTIUM 3 PROCESSOR. I RECENTLY SAW ON A WEBSITE THAT THE MAX THE MOTHERBOARD CAN HANDLE WAS 800MHz. I DONT KNOW IF ITS TRUE SO I NEED YOUR GUYS HELP. THE PROCESSOR IM THINKING ABOUT IS A 1GHz CELERON WHICH WILL COST $45 TOTAL WITH SHIPPING. BEFORE I BUY IT CAN YOU GUYS HELP ME. EMAIL ME PLZ WITH YOU INFO ON THIS AT res18zzb@hotmail.com.