They had to sacrifice some textures due to the additions and the amount of ram they took in-game.  It was new ships and abilities-particles or multiple copies of planets.

so sinperium what would make it possible for them to use ALL the power of the cores? or what would they have to do to add more stuff and not worry about slowing everything down?

dose it take a long time to upgrade to 64?

They would have to rebuild the game from the ground up.

Look at all the bugs, minidumps, desyncs, etc. they deal with now.  Moving all that code over opens the whole can of worms back up.  Sins code seems to chain react pretty easily--one thing tips three others or breaks it all, etc.

To do a through job and really proof it would probably require about the same amount of time as just completely rewriting it--so you are talking about developing an old game instead of making new ones.  

Since Rebellion has already taken a lot of time, I can't imagine that's possible now (not to mention they already said it isn't).

I'd hope that Sins II will get made one day in the NOT too distant future and they might add things in like stock markets, more setup and map design options, more customizable features, etc. to really broaden the game out--not just dub it over as is.

But for now, here we are at 32 bit.  Mods are the answer to your texture questions.  Using a TSOp-lioke approach where you cut out higher resolutions carefully and redundant and over-sized textures, you can probably squeeze a few more in.