VSync ON can hit FPS really hard.

Depending on what the game does at the moment, the updating gets done in X milliseconds and the rendering in Y milliseconds. The FPS you get is 1000/(X+Y).
If your FPS is 50, X+Y equals 20.

When VerticalSync is activated, the card won't start displaying the image until a new screen refresh starts. So when in 60Hz, on average, the render takes a 1000/60/2=8.3 milliseconds longer so your new FPS will be 1000/(X+Y+8.3).
Starting from 50 frames per second, you get 35 with VerticalSync.
That's on the average. In the worst case scenario, your FPS will drop to 27.

It's a big hit on FPS without any real visual improvements.

As seen from the formula, it helps a little if the refresh is set to 75Hz. The 8.3 milliseconds delay takes only 6.6 milliseconds and the FPS drops only to 38.

This low FPS can be the cause of your video card running in "2D" frequencies. The video card driver may say "this game is very CPU bound, instead of GPU bound, so the card will do great even with the 2D frequencies and the player will notice the reduced fan noise".

In Silent Hunter, switching the VerticalSync requires you to restart the executable (to reinitialize the Direct3D engine).

you just need to make sure you get more fps than your screens refresh rate, then you wont have such problems.

my screen is 60hz and my fps stay at 60 all the time with vsync enabled.

and yes vsync does a lot, it gets rid of tearing, but if your frame rate is lower than your refresh rate there is no need for vsync as it chops your fps in half and you dont get much screen tearing with low fps anyway.

screen tearing is generaly more noticable when your frame rate is higher than your screens refresh rate.

with my ati1800 i get terrible tearing with
vsync off even at lower framerates.

Indeed it is. So long as you have paired chips (ie, an even number) it will run in dual channel mode. However, if the chips are not the same size, you'll only be "dual channeling" an ammount of RAM, equal to the smaller of the DIMMs.

Prolly not a big deal in practice, but theoretically, it could make a difference. Or at least that's what Sandra says ;)

yer i have the 2 1gb sticks paired up in their colour coded slots and the 2 512mb sticks in their slots, everything reports 3gb and dual channel mode, tests show the same

Im not convinced the problem has been fully isolated. I recommend you download 3dmark and compare your benchmark results with similar hardware to better understand what your rig is doing.