What comprises the bottleneck depends, really. On my rig it's anything BUT the graphics card. I recently bought an ATi X1650 Pro to replace my Radeon 9600 Pro. A fine card for the 77 Euros I spent on it, but it's limited by:
a) the motherboard which only supports up to AGP 4x,
b) the P4 Northwood running at 2.6 GHz overclocked,
c) the 1280MB of plain DDR RAM PC2600, and
d) the PATA hard drive.
But yes, I agree with you that what one might find enjoyable the other might find appaling. I'm fine with the 20 fps I get with most of the settings cranked way up and a mild 2xAA and 4xAF...
...unless there's stormy weather...and/or multiple ships...
But eh, I can still play the game and enjoy it!