Krag, the key are the memory dividers such as 3:4, 4:5, and 4:6. I think the motherboard manufacturers are starting to get the idea we want all of these above 133 FSB. Better yet, infinitely adjustable in 1 mhz increments; I can see that coming.
When I run my CPU at 160, my ram is running on a 3:4 divider, e.g., 160 x 4 divided by 3 = 213 mhz, or DDR426 (check the Sandra screenie). Even when I run 165, 1:1, I only get around 2500/2400, so what you are getting at 183 FSB sounds about right.
I am fairly surprised at my scores though, as my P4B266-E board is locked out at 2.5-3-3-7 above 140 FSB. I'm pretty sure I can do 2.5-2-2-6 at DDR400-420 so I would get even higher scores if I could adjust the settings. But I'm using genuine Samsung ram which I burnt in well.
Note at 155/DDR412 I can do it using 2-256 MB sticks, but at 160/DDR426, I can only do it with 1 stick. More importantly, both setups are totally stable in 3D. I have to hack my board and trick the BIOS into running the 3:4 divider, so I have to use jumpers to set my FSB. We can only do 155 and 160 now, so I can't try higher. But I doubt I can do much better than DDR426.
What MB are you using? You may be able to set some dividers other than 1:1.