Frankly, if you're asking a question like "is this program hacker-proof", you've already lost. What you should always focus on is specific attacks (which you did in the body, e.g., "limit on post to prevent buffer overflows" and "secure from SQL injection"). The general statement of "is this program secure?" made in the title is far too naive to be useful.

Here's a great example of securing an application, this one involving ATMs: http://www.theregister.co.uk/2004/07/21/atm_keypad_security/. Notice how very specific requirements are made ("an attacker must take the PIN pad home and spend at least 10 hours breaking it").