My PC is now pentium(R) 1.8GHz. I would like to change my processor to the one perform more faster.My badget is not high so i prefer core2duo 2.0GHz or higher. The problrm is i wonder if my motherboard support the processor or not, my motherboard is baught by the time with processor pentium(R).Or should I go for pentium(D) 2.0GHz or higher?
I'm pretty sure that any Pentium 4 processor clocked at 1.8 GHz uses Socket 478. Core 2 Duo processors use LGA775, so you will need to change your motherboard and RAM at the very least.
I don't think there is much point in upgrading just the CPU in a setup like yours since the small performance increase is not worth your trouble.
I don't know what your budget is, but a Pentium E5200 with a decent compatible LGA775 motherboard and 2 GB DDR2 RAM shouldn't be more than $200 total. If you want a Core 2 Duo and not a Pentium Dual Core, you can get away with a $250-300 budget. The Core 2 Duo E7400 is a good CPU for the price.
Consider getting a motherboard with integrated graphics if you're not much of a gamer, that way you won't have to spend extra on a discrete video card.