iOS What are the image resolutions for iphone3g, iphone4, ipad?

[webphone]

I would like to build a universal app but I am not sure the image resolutions for iphone3g, iphone4, ipad?

iphone3g - 320x480
iphone4 - 640x960
ipad?

Or not change the dimension of an image, but change its ppi?

Please advise

 

[cnstoll]

http://developer.apple.com/library/ios/#qa/qa2010/qa1686.html