You have a Slot A motherboard. The fastest CPU made for Slot A was 1GHz, and that is very expensive and rare to find now. I don't think it'd be worth changing the CPU.
If you want to go to 1.8GHz, you'd have to replace almost the entire computer.
What do you want more performance for? If you've only got 128mb and WinXp, adding another 256mb would help alot. If your hard drive is only 5200rpm, replacing it wth a 7200rpm drive would be lots faster. Your CPU would probably overclock to 105MHz x 6.5x = 682.5MHz