How big can I print a 1200x900 pic?

Hi all,

I have taken his pic, which I really like and I would like to print it as big as possible, but it should still look nice... (i.e. not pixelish, I think you know what I mean...)

So I am wondering - the original is 1200x900 (without the added frame, just the image) pixels as it's a cropped image (and that's a 100% view), so:

1) How big can I print it? Should I assume 300dpi, so it's something like 4x3 inch?

2) Can any "upscaling" be done (is that the right word), so I can print it bigger? (Not sure if there is something like that as I thought that you can't get more image info than is there in the first place, but I thought I'd check anyway)

I don't know too much about printing and dpi stuff (you might have guessed ), so please be kind to me

Many thanks in advance!

Cheers,
Pablo