MadShrimps has a handy guide to overclocking your X1900 series video card. Here's an excerpt:

The memory is easier to overclock because it does not hang up your system or crash if you have gone too far, and if you are quick enough (you will understand this statement once you begin experimenting with the memory).

For the memory I suggest you start off with a 20MHz-25MHz increase initially (using the same steps as above for adjusting the Core frequency, but now you are dealing with the Memory frequency). Once you have made the first jump, let the 3D cube heat up your video card to maximum heat. Once it reaches the highest temperature possible without artifacting, close the "3D View" again and increase your memory clocks by another 10MHz. Repeat this process until you begin seeing artifacts on the 3D cube.

Here is where you have to be quick: if you can close "3D View" fast enough upon spotting artifacts from overclocking the memory on the cube, you can avoid having to reboot your system, and you can lower your memory frequency to what it was before, then you try again (using a smaller increment this time.

This may generate some artifacts on your desktop, but don't worry. They can be easily eliminated by changing the resolution of your display (I usually set it to a lower resolution then I refuse the changes and that takes me back to my original resolution) or by restarting.