Haha it does get a bit much, if we include non-FPS games there must be more than 5 games just called AVP. AVP (Classic 2000) is great though you're in for a treat. So is it's sequel AVP 2 which is the only main entry in the AVP FPS games not made by Rebellion afaik, it's made by Monolith. The Jag one is the very first one yeah and it has an engine like Doom which is cool (the enemies are 2d sprites and whatnot, also like Alien Trilogy in fact) and even back then it had great atmosphere. A collection of all these in a bundle would be great, AVP Jag, AVP Classic 2000, AVP 2, and the most recent AVP.
Alien Trilogy and Alien Resurrection are great games too though now you mention them! Playing through Resurrection again at the moment actually. I'd LOVE to make an FPS with a low-poly, pixelated PS1 style but with cool lighting and atmospherics - and Alien Resurrection is exactly that. Shame about the film though!
Acid deaths from close-range headshots are a problem. I mentioned it before but the key on Nightmare really is to abuse the heck out of the melee system if they're close. If they're winding up a big unblockable melee attack, whack 'em with RB, otherwise block with LB and RB and when they use a fast attack, RB them when they bounce off your block. Both ways will end up with them on the floor at a safe distance struggling to get up, a sitting duck! It neutralises the threat so well you can just use the pistol a lot of the time if you fancy saving ammo. You can use it just to buy you space and time too. I just finished a play through and it still works like a charm!