I am pretty new to this stuff myself, but I will try to answer your questions.
1. DDR Vcc (VDIMM) - this can be increased as the FSB is increased, should the RAM become unstable. Given that you have DDR400 ram, which can be run at 200MHz, it should not be necessary to do so, provided that you don't go much beyond 200MHz.
2. Yes, the multiplier is locked.
3. CPU Core voltage (VCORE) - This can be increased to restore stability as the FSB is increased.
4. As I understand it, yes, auto should be fine for you to run your DDR400 ram with FSB 200MHZ.
5. Memory timings - Unfortunately I have not had luck changing this from Optimal to anything else. I too have Corsair Value select RAM 512MB, but it is supposed to run at CAS 2.5.
6. Yeah, I would like to have that answered too...
As additional info - I have a 3000+ Socket A on an MSI K7N2 Delta2 LSR mobo that runs at 200FSB. CPU interface - Optimal; Ram - Auto @ default VDIMM; VCORE 1.8V.
For more info check out this link Overclocking Guide