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