What causes the "access" times?

This is probably a very rudimentary question, but I'm wondering what would cause the game to stall for a half-second when I change a weapon, open a door to a new room, or encounter a new enemy (seems to happen more when its something that "hasn't been displayed for a while", for lack of a better description).

Does this mean that Doom 3 is having to use a swap file? My limited knowledge makes me want to come to this conclusion, but I have 768MB of RAM with a 400mhz FSB, and when I check my system resources when I'm running Doom 3 it never shows the program using more than 400MB, and the non-Doom 3 programs aren't using more than 50. Perhaps it has something to do with the video RAM then?

Anyway, I'm interested in figuring this out both out of curiosity and because I'm about to build a new system and want to avoid this (or more specifically, to have a perfect Doom 3 machine).

Here are my complete specs:

Pentium 4 2.4Ghz
Soyo P4X400 Dragon Lite
768MB RAM
Radeon 9800 (non-pro, 128MB)
Windows XP Professional

Thanks for any help.