karma etiquette is very fluid. you can give karma for any post that you like, for whatever reason you like. it's just a way to show your appreciation. you can also choose whatever amount you like (up to your maximum available, which shows by default in the karma dialogue). there's no rule about how much you should give, you can decide that for yourself, and other than the minimum of 2 points set by the system it's up to your discretion.
i think generally people are pleased to get karma regardless of the amount, in fact a lot of people value the small karma hits from new people because they usually indicate that we were able to help someone out. and also, i doubt most people are keeping track enough to figure out exactly how much each person gave, so no worries.
as for seeing who gave you karma, click on "user cp" in the blue navigation bar at the top of the page, and scroll down : you'll see a table called "latest reputation". this shows you the post you got karma for (it's a direct link to your post), the name of the person, and the comment they left you (if there is one). the person's name is also linked to their profile page, so you can click right over to say thank you.
karma in a nutshell !
there is also a whole thread
explaining karma which you can peruse if you like to learn how karma is calculated.
i hope that answers your questions.