Are there any 400+ watt psu that fit Gateway GT4026E desktop computers? For a Gigabyte Geforce 8800GT?

I recently bought a Gigabyte geforce 8800 GT for my desktop and it requires a 400watt power supply to the card. My current setup is an insufficient 300 watt psu which isn't nearly enough to run the badboy. Are there any 400-450 watt psu's that will fit the gateway GT4026E model? The dimensions are 4 x 6 x 5.5-inches. Any $50 or under? Will the Dynex atx 400 watt work?