I would suggest 300dpi.

It depends on what you want to do with the scan. Your scan should be at the dpi that is correct for how you will use it.

Let's assume you want to scan an old 4x5 inch photo so you can reproduce it at the same size on a modern inkjet printer like an HP Photosmart:

To start you need to know the dpi requirement of the printer that will give you the printer's best result. An HP Photosmart printer will max out with an input image around 200 dpi -- the same is true for most of the major brand inkjet printers. In other words dpi above 200 in a file sent to an inkjet printer is overkill -- it does no good.

(You've been scanning at 150 dpi. The difference between 150 and 200 dpi is pretty minor and most people aren't going to notice it.)

So, if you want to scan a 4x5 inch photo to output on an HP Photosmart printer at 4x5 inches a scan at 200 dpi would be ideal.

But what if you want to scan a 4x5 inch photo to output on an HP Photosmart printer at 8x10 inches? If you scan the original at 200 dpi you will lose half your resolution when you scale it up to 8x10 -- it will only be 100 dpi.

So, to get an 8x10 inch 200 dpi file to send to a Photosmart printer you need to scan your 4x5 original at 400 dpi to account for the scaling change. You see it's a direct mathematical relationship: 5 * 2 = 10 so 200 * 2 = 400.

shouldn't that be 800dpi for an 8 x 10 seeing as the scan area will be 4 times the area of a 4 x 5 not 2 ?

Nope, that should be 400. You're right about the area increase, total pixel count and file size will quadruple, but dpi is a linear measure. If you have 200 dots per inch and ten inches then in a ten inch line you'll have 2000 dots. If all you do is squeeze the 10 inches down to 5 without adding or subtracting dots you'll still have 2000 dots: 2000 / 5 = 400.

Thanks firewire800