RAM, maybe, I think some people have done this but you need special tools(reflow I think).
There is also a "Hack" where you replace the wireless Mini pci with a broadcom card for viewing 1080p content.
Thanks, I heard of that hack too and I was wondering if you could use that to upgrade the GPU.
Depends, what do you want to do with it, if it runs AppleTv OS and some fiddling with the software it will play HD.
I have one and installed OS X on it but it is slow.
Oh yeah, it's called CristalHD