Upgrading My CPU for Dell Dimension 4500

Hey Folks i currently want to upgrade my CPU but i not sure which one will be acceptable for my system and will i have to change my motherboard or anything like that.. I will display my system details below

Win Xp
Pentium 4 CPU 1.80GHZ,
1.79GHZ 768MB RAM
Sapphire Radeon 9550 256MB DDR Memory Graphics
I’m not sure what my motherboard is but its the basic factory one that comes with Dimension 4500. I dont think i can upgrade my Ram to anything more. i think thats the max for my motherbaord.. ! Much help and info would be greatly appreciated.! Cheers folks ! Dan