What is the difference between kvr533d2n4 and kvr667d2n5 1gb memory?

Not much difference. The first one has a slower clock speed and a latency of 4. The second one has a faster clock speed but a latency of 5. So the true speed works out to be almost exactly the same.

The one difference is that the second one has a higher bandwidth (5300 MB/s versus 4200 MB/s for the first one) which will make it perform somewhat better if your motherboard supports it. If your motherboard doesn't support anything more than 4200, the two sticks are identical for all practical purposes.

Since the price difference between the two is only 50 cents, you might as well get the second one.

What motherboard do you have, by the way? You may be better off getting something else for just as cheap, depending on what the board can support.