Print big prints

I shoot raw with my 400D with has 10.1Mpix. I just realize that if I crop some of my photos to a 8x10 on cs3, the PPI(DPI) are under 300. What i do is to manually adjust the PPI (DPI)[Image size] to 300. I don't know if it is suggested .

Then also I have a though that if I crop my image to non- 3:2 ratio (e.g. 8x10, 11x14), it will lose lots of PPI after crop and can't reach the 300 ppi recommended by most printers. what do you do under that condition? I know some of you can make prints up to 20x30, will the prints reach 300dpi?

Thanks a bunch/

Use software to extapolate your images to whatever the native resolution of your printer is. Qimage is a good program to use for this since you don't have to permanently resize your original files.

Forget about ppi and dpi, just process the images how you want them and send them to your lab. Not many labs require a specific ppi, they will usually interpolate for you as necessary. I make images up to 30x40" from my 20D, it falls below 100ppi but from the usual viewing distance they look great - you don't look at big images from 30cm away usually.