I'm having nikon d40x and nikon d5100?

Which memory card should i use in sandisk SDHC ultra or extreme? Please tell me

All depends. With a camera apart from the size of the memory (gigabytes and so on) the important thing is the speed that it can be written to.

If you are planning on taking lots of shots one after another then you need a fast card (generally most expensive for the size). If you take less than about 1 or 2 shots a second then any card will be OK.

The D40x will work fine with nearly any memory card. It does not take photos at a high enough resolution or at a fast enough rate to require any more than a class 4/class 6 card.

Your D5100 may be able to exceed class 4 data rates, though; especially if you tend to be loose with your shutter finger. If you shoot faster than your card can store images, it won't hurt anything; it'll just cause your camera to stop working for a few seconds when the buffer fills.

Most people can get away with shooting with a slow, cheap memory card. Remember that if you are filling your card at a rate of 10 MB/sec, it'll only take you 15 minutes to fill an 8G card. How many photos per minute do you typically take?

If you shoot video, get a Class 10 SDHC card. For pictures, any will do.