I have problems with my GeForce FX 5600 too
It seems all my games are laggy even Vice City wich ran smoothly on my GeForce 3 Ti 200.
My configuration is
AMD Athlon XP 1800+
256 DDR 266
HDD Seagate 60 GB
Leadtek GeForce FX 5600 128 MB
Motherboard ECS K7S5A
What enrages me is that a friend of mine with the same config as me and with a Radeon 9600 doesn't have this problem

Do you have the latest drivers? And check you computer temperatures and make sure there not to hot. This will be a good starting point. If this doesn't help let me know and explain the problem in a little more detail and I'll be happy to see if there is anything I can do to help.

Yes i have the latest drivers the processor temp is 55 degrees and the video card is 67 degrees. And well for example in Vice City the game is runing like a slide show almost and very few games actually work good

What version of DirectX are you using? I would also try to completely uninstall the video card and the drivers (This program will help)
and reinstall them see if that helps.

I am using DirectX 9.0b
And the program you sugested doesn't work it says unknown card or non nvidia driver
P.S. i am using Windows XP Professional

Are you using third party drivers?

ok you lost me a bit what's a third party driver?

Did you get your drivers from Nvidia's website?

yes i got the driver from the nvidia site

I have had all these same problems with the geforce FX 5600 256MB. I did everything talked about in several forums, and nothing fixes it, except changing to 2X, but games like Battlefield 1942 crash all the time with the 2x setting. My board only supports up to 4X. I came to the conclusion that the geforce fx 5600 simpley does not run correctly at 4X. So I decided I would have to find a card that would, or upgrade my system. I went from an Asus A7M266-D (I miss my dual processors) to an A7N8X-Deluxe, and the problems went away. The card just doesnt like 4X. So if you are running on AGP 4X buy a Radeon! Freakin INVIDIA!.... Oh, but I love the card now.