Aliens is likely going to look ass, with the grain being part of the raw footage - Hi def grain, woo! Also, what's the bet wires are going to be popping up all over?
Yes aliens is a very grainy movie, but so is predator, and while it cant hold a candle to new productions on blu-ray it still looks miles better then the DVD version. Ever took a look?
And i dont just talk about about some screencaps here, watch the thing in movement on a giant tv or beamer, it looks ace.
It really does look far better, i never thought it myself, i was just like you "How can this movie work on blu ray, its going to look like shit". But no, really, it looks great on blu-ray.
I don't see why aliens should make a difference, again, it wont look as stunning as new productions, but still great i guess and worth the upgrade.
If you don't believe in blu-ray or don't think its worth the upgrade then skip it and watch the DVD, no one forces you.
But i personally enjoy the new quality for what it is, and there shouldn't be a problem with effects as well, Predator's effects look just as great as if you were watching them in standart definition.