^ Raid makes everything that uses the disk much faster. period.
As far as loading game levels and the like, it depends on the game. A big part of level loading is that the CPU is compiling the level for your use- not just loading the game from disk. Some games require more CPU usage before entering the level, some are more just waiting for the level to load from disk- depends on the game.
All I know is that, without exception- every single time for months now- I am the first to enter the newly loaded levels in BF2 by a pretty big margin. Those servers that don't have time limits before the game starts are where this makes the biggest difference- because I am capping flags before the second guy of the 64 total makes it into the game. So, YES, RAID makes a big difference (in addition to a fast comp, of course).
If you go on that crappy RAID review that anandtech did that tested one raid controller, on one game, then made a blanket statement "RAID isn't worth it and doesn't speed up games" - then I can see why you'd think that. It depends on many factors - the main one being how much money you are willing to spend on your equipment.