Installing cpu upgrade.

I want to upgrade from my celeron 850MHz to a faster cpu. Preferrably 1 gig or higher. But I want to make sure that the ASUS motherboard in my HP XL916 will support it. Also has 256 MB of RAM. Chipset with this system is 810. Does that make any difference in the cpu upgrade? I am confused,and need help. I don’t want to screw this up. What exactly do I need to know to be able to match the CPU with my motherboard?