return vents

I live in a 3br cookie-cutter 50's ranch. I currently have one large baseboard register (30x8 inch, maybe) in the center of the house in a hallway and one in the livingroom (16 x 8) None of the bedrooms have returns. The house is about 1500 sq ft. I currently get negative pressure issues...

So I'm living in a 2 story house with a basement built in 2001. The house has 2 AC units one for upstairs, then other for downstairs and basement. This is a rather large house and the cooling/heating bills are more than what we'd wish for. The top of the house is the hottest (not by much) as one...