Aert or Small pop-up

you can either use response.redirect("<script>alert('Your text goes here')</script>")

Or you can use RegisterClientScriptBlock("key","<script>alert('Your text goes here')</script>")

both are equal but sometimes response.redirect changes the way the page is rendered on clicking ok.
I would suggest the second option.

you could also use conirm instead of alert.this would give the user the option to click (ok and cancel)
Depending on which you could perform further action like clicking on ok you might like to redirect to the updated page like it happens on facebook notification.