I still have not figured out how I'm better off with Lion on my old Core2 Duo MBP. I'm switching back to SL. Maybe the older OS will let me boot faster than 1.5 minutes.
I have a Core2Duo iMac from 2006, and it runs Lion beautifully with 3GB*. Lion has a lot of nice touches, particularly in the Finder. Copying files has new features, including the possibility to rename rather than overwrite.
Spotlight has easier ways of using search tags, i.e. "Emails from", "Date Created", "Audio bitrate", etc.
Personally, I love Resume: I switch off my Mac at night, so it's nice in the morning to turn it on and have everything restored. Auto-save and Versions is great, too, though I have to remember which apps do it and which I have to save still! It's a major change in the way you think about files, which some people can't get used to.
There are many little improvements. (Though a fair few bugs as well.) If you really want to know more, the Ars Technical review is very thorough.
http://arstechnica.com/apple/2011/07/mac-os-x-10-7/
If your machine can't cope with 10.8, then I would move up to Lion. Otherwise, you might as well skip it and get Mountain Lion.
* Lion seems to use as much memory as you have. Lots of people have cried out "OMG where's all my Free RAM gone". But memory is there to be used. Nothing wrong with an OS that uses all your RAM.