Quote: "That should have been read as meaning "small environments", as in there is literally very little room to move around because the rooms are so tiny."
You know you can get out on the martian surface right?
It's bloody vast... you have to skip between certain areas like that because the airlocks and such have been destroyed.
As for the physics, again wait until later. The game eases you into everything really, it starts of very simplistic but then grows in complexity of what you have to do.
Remember the Environments are all self contained, although the Zombies are mindless morons, the small spider things and cerub things are danm simple to kill but they learn each time they attack you.
Once you've got to the other end of the rail-system you realise this as they start setting up traps for you. It is scarey when you follow one down a hall to off it and come across 20 of them which have obviously been waiting.
The start of the game really is there to make you feel trapped, clostraphobic and ease you into the story.
One that is gone you are just quite frankly attacked randomly, before the real fun begins... Monsters won't attack you but lie in wait in corners, i actually though the AI was broken on one of those big ugly dudes as i shone a tourch at it, didn't move so i move a lil close, until i was right infront of him. Thought he was some kinda statue, turned my back and hear him roar in the surround.
Tell you that REALLY does scare the crap out of you. I mean sure you could just rush through the game and sod the story, you'll notice it does appear to be extremely linear and simplistic.
But when you start tinkering with your environments and looking around, yeah it does certainly get interesting.
As for your graphics problem, make sure your AGP Apature is set to 128MB, also it's the textures which take up the Ram, so you set them to medium and you should be alright at most resolutions.
I've not been hearing good things about performance of Doom on Pentium4 Processors though.
You could quite easily pickup an Athlon64 3000+/Mobo/1GB PC2100 for around £250ish
which is worth it imo with games like this and half-life 2.