Lag with good fps

Here are my specs:
Core i7 920 @ 3.8ghz
MSI GTX260 SLI OCed
Asus p6t6 WS Revolution
Corsair Dominator 6gb 1600mhz
Thermaltake Toughpower 1200w
Seagate Savvio SAS hard drive
Samsung 2433 24" LCD
G9 Mouse
Windows 7 RTM x64

I get great FPS when playing Crysis Wars on low (100+) but when I up the settings and go below 100 FPS I experience this extremely annoying lag which seems to be input lag. The game itself looks very smooth but the mouse seems very slow/laggy. I have messed with the nvidia control panel, my mouse settings in setpoint, changed the drivers numerous times, and messed with vsync. When the FPS is capped with vsync the lag for me is almost unbearable. I have tried everything I can think of to correct this problem and read numerous forums to no avail.

Reinstalled the game?

I've just bought a new PC to replace my old struggling PC.

I am experiencing similar problems with most games I play. My old PC - which had an ATi card - never ever had input lag, as far as I recall. But for some reason, the new one (nVidia card) has absolutely disgusting input lag regardless of whether the game is performing well or not.

I've spent a lot of time trying to find a solution, and this is the best I came up with:

-> Increased USB polling rate from 125Hz to 500Hz (negligible difference)
-> Set "Max Prerendered Frames" in nVidia control panel from the default "3" down to "0" (this worked! Half Life 2 was immedietely very responsive, but could be jittery at very high res + high detail settings)
-> Experiment with Triple Buffering on/off - I found some games override this setting anyway, but worth a go
-> I installed Windows XP SP3 on a seperate partition, and immedietely noticed a huge improvement over Vista. I can't offer any explaination for this.
-> Capping the FPS manually in game is suggested in a lot of forums - fps_max 59 (on a 60Hz screen) in Half Life 2 for example. I noticed no improvement personally.
-> Sadly, the only thing that completely elimited all lag was disabling VSync. So now I have to live with excessive screen tearing that makes you want to vomit - but I'd prefer this to input lag.

Strange thing is, even though my old PC (ATi Radeon 1950 Pro I think) was far far inferior, and got much lower frame rates, input was always extremely responsive, Vsync or no Vsync!

Let me know if any of these work, or if you find any other solutions...