EggXpert

Is it a slim tower like this one? Please confirm.

http://us.gateway.com/gw/en/US/content/model/PT.GBS02.001

With the 65W Core 2 Duo, you don't really need 400W but .......

This Seasonic is a decent quality PSU and may well fit in the box.

http://www.newegg.com/Product/Product.aspx?Item=N82E16817151090

Please look at the product dimensions and pictues on NewEgg carefully and think how the unit would be installed in your computer case. Check things like whether there's enough space inside your computer for this PSU to fit, where the flip switch is on this PSU and if your case has a hole for the switch so that you can flip it from outside the case. The fan on the PSU should not be pressed against a computer case panel, so you may have to flip the PSU upside down in your case. It should be fine but just in case, please check out the locations of the screw holes etc.

Unfortunately, there aren't pictures of your case shot from multiple angles online or pictures of the interior of your computer, so I can't check those things, but you have your computer right in front of you. I am sure you are in a better position to picture how the unit would fit given the availability of PSU pictures on NewEgg.