I agree with Bub - 'Copy Merged' is the best option for a snapshot or, if you want to destructively edit something, save a new copy and crop it that way (so if anything goes wrong, you can just go back).
PS. If you want to maintain layer effect ratios, dont forget to scale them in proportion to the decreased size of the image. I'm pretty sure I have a tutorial on this somewhere... If not, let me know and i'll make one this friday...
