Midgard has it spot on.

Regarding the bit of text thats confusing you from post #4 "A4 which is 8.3 x 11.7 then at 300dpi that means images much larger than 2500x3500 are a bit wasted" then what I was saying is that A4 is exactly 210x297mm which is approx 8.26 x 11.69 inches. If you use 300 dots per inch then thats 300 x 8.26 dots wide or 2478. For the height its 11.69 x 300 which is 3507. So if you hold a piece of (good quality) paper in front of you without getting right up close it to or using a magnifier, then any image with more than (2478,3507) pixels cannot be printed with any more visual detail. If you plan on getting right up close to it as if you were printing a bit of card for a model that you might look at really close and your paper can cope (which it wont) then maybe you could use 600dpi in which case the image could be up to 5000 by 7000 pixels. Or you could print an A2 image with that same 5000 by 7000 pixel image at 300 dpi.

You can note that if you only ever plan on printing A4 photos and never hold them up to your nose then any digital camera with much more than 8.75 mega pixels is just burning hard drive space for no reason. Theres other reasons why very large megapixel consumer digital cameras are a dumb idea but thats another topic.

Also note, when your printer says 1440 dpi or 2800 dpi or something equally more than 300 dpi that has an impact on dithering more than the colour dpi of an image. When we say 300 dpi we mean 300 dots of any colour per inch. When your printers says 1440 dpi it means 1440 dots of one of the ink colours which is usually one of 4 or one of 6 only and it has to dither it to get a full colour pixel at MUCH less than 300 dpi. You probably wont be able to print at 300 dpi on any normal ink jet printer you can buy but its the setting of the 300 dpi param in the photocopier / printer driver that you need to get to grip with in order for it not to print 4 sheets of A4.