IF Function Problems with Excel 2007 - Help!

Hello! I'm new to this forum, but I need your help. I'm adding functions to a spreadsheet for work, but I am running into some errors. Basically, I need the functions to do the following.

1) If E5 is blank or 0, F5 needs to say $0.00.
2) If E5 is <6 but >0, F5 needs to say $450.00.
3) If E5 is >=6, F5 needs to say $900.00.
4) All of the results from the F column need to be calculated into a total dollar amount, i.e. $1,800.00.

I've gotten functions two and three to work with the IF Function (see below). However, it always says $450.00 when the cell is blank or is at 0.

=IF(IF(E5<6,AND(E5>0)),"$450.00","$900.00")

Also, when trying to use the SUM function on the values in column F, it says 0. I believe that is because the IF Function produces text values instead of numbers.

How can I fix these errors? Thanks so much for your help!

Lesley