how to enable fotostream on an mac book pro 15'' core2duo older than mid 2007?

hi

I have an old mac book pro which I like to reuse as a family share for music, movies und photos.

I'd like to make a fotostream as a family album, everybody in our family can share their best pics and the mentioned mac book pro hosts the stream and downloads the pics in iphoto, backups via timemaschine and so on.

unfortunately the appstore won't let me upgrate to snow leopard, my book seems to be to old, all other requirements are fullfilled, so i think it might be too old.

this means i cannot buy the new iphoto at the moment the book has iphoto 8.2.1.

Has anybody an idea? or is the only way to buy a new mac?

thanks for your help!

kristin