I believe about the largest drive you can do internal (without surgery on the PS3) is about 750GB or so. The 1TB 2.5" drives are a little thicker than the smaller capacity drives and will not physically fit.
Personally, I would wait on the external and see what the near future holds for NTFS support. I think most games will fit on external just fine with a few exceptions. The ones that don't do well on the external could be placed on the internal.
Depending on the size hard drive you have right now, you could go ahead and buy the 1TB and do what I am saying above.
I have a nice, fat hard drive waiting in the wings for NTFS support.
I would much rather go to my grave not needing my gun, than go there wishing I had it.