Do you think i can ask under "How did you find us?: 1. Search engine listing; 2. Search engine ad. Or would you just use 1. Search engine.

I've been doing this for years. We typically ask "How did you find us?" and then provide a dropdown with the most common methods used to find the site. We will modify those dropdowns periodically based on statistics.

I do have Google conversion tracking on but i thought it would be good to separate it at this point as well if i could get accurate results.

There won't be any real accuracy in the results submitted by the visitors. They tend to forget what term they used when they found you, especially if the site is engaging and it takes their mind off that original search query, happens all the time.

After all these years of implementing this on contact forms, registration pages, etc., we've decided on one format...

How did you find us?
Search Term
Other

We provide an "Other" option in the dropdown so the visitors can specify what "other" method they used to find us. If we find we get enough "other"s that are identical, we'll add that to the dropdown.

The easier you make it for them to select from a predefined popular list, the more accurate it becomes. But, it will never replace raw statistics. ;)