Depends on your cooling and your stepping. The old dual core Pentiums are rather leaky. You may hit 4.0 on air (with an after market cooler of course). Then there is liquid cooling, and on the extreme end liquid nitrogen. Are you just trying to overclock it for fun at the risk of blowing it, or do you want it to be your daily gaming machine?
This is my daily gaming machine. I am going to run 3DMark on it later tonight since it seems that's the standard around here, but I think my CPU is bottle-necking me. With the Zalman Heatsink in never gets close to hot.
Yeah, that CPU is bottle necking you all right. Anyway, for the Ratios it's best to start off with a 1:1 ratio. If you leave your RAM timings to auto the motherboard will generally lower them so set them manually as loose as they will go. After you have settled on a final FSB speed, then you can begin to change the ratio in favor of Faster RAM and try tighter timings. Of course, use memtest and other stress tests to ensure it's stable ^_^. Anyway, read some overclocking guides for more info on overclocking the old Pentium Ds.
Just so you know, even overclocked, that CPU will still bottle neck your GPU. Still, at least overclocked it would be more bearable .
If you set the Ratio at 1:1, then yes. As you increase your FSB speed, you will increase the speed of your RAM as well. Once you can't raise your FSB anymore, then you can change the ratio to see what you can get out of your RAM. You do it this way to ensure that your RAM is not the limiting factor in your overclock.