Upgrading OS slowing down machine How does one know if upgrading OS will slow down their relatively older machine? 

For example I bought a MacBook Air a couple of years ago. It came with Leopard and I've already upgraded to Snow Leopard, but now want to download Lion. I'm concerned though that it may slow down my machine. 

I've heard of this before where upgrading OS slows down performance on older hardware.

How can one know if they risk a slower performing machine if they update OS?
Lion seems to use a lot more memory than Snow Leopard. I had to upgrade my 2 year old iMac from 4GB to 8GB of memory, and it still feels slow sometimes. You may want to avoid the Lion upgrade if you have less than 4GB of memory, and definitely if you have less than 2GB.