Here's my theory (and this rings especially true with added texture packs):

When playing a non-open-world game such as... let's say a Quake deathmatch arena, the entire level can be loaded in RAM at match start and not have to change.

When playing a game like FO3, which never knows what direction you're going to spin around in and start running in at any given moment, it has to constantly head to the HDD to load up new textures.

As such, isn't it possible that the single greatest bottleneck to FO3 is actually ones hard drive?

I upgraded my video card from a 9500 GT to a GTX 460 and there wasn't that big of an improvement. I also hear my HD clattering much more than most games cause it to.

With these massive open-world games from Bethesda, cell-rendering is rather frequent...so yes, the hard drive is certainly one of the factors for how much stutter and load times you'll see. Oblivion was one of the worst games in this regard even today. My take...have no less than a 7200RPM hard drive with at least 16MB cache. My 1TB Samsung F3 runs these games relatively smoothly for me. Others have indicated that SSD's gives a great improvement in these games, but I still can't find it in myself to pay so much for so little capacity.

And there should be mods now to help relieve the performance issues.

But is your DATA folder 35 gigs full of data like mine:P?

I have about 10.5GB in my Fallout install folder. I have it installed to the SSD, where I find it really helps loading times. I had it installed on my velociraptor, where it was running fine, but the random stutter when running in the wasteland almost completely disappeared when its installed to the SSD.