Is the Voodoo lagfix compatible with Darky's ROM? My SGS has awful lag, and I'm considering my steps.  

The first step I want to do is install Voodoo, because I'm under the impression that it's relatively easy and will not require my to reinstall everything on my phone (though I asked about that in a related question to make sure).  If that doesn't provide the desired results, I want to flash a custom ROM.  The one I'm looking at is Darky's ROM.  

Will I have to uninstall Voodoo before installing Darky's ROM, or can I install Darky's on top of Voodoo?
The default kernel (Dark Core) is derived from the Voodoo kernels, but the lagfix is disabled by default. For example, the v9.5 description on XDA has the following note:


  New Kernel >> Dark Core v1.0 inside! I9000, I9000M, I9000T ONLY (Cappy users choose speedmod or skip kernel)
  
  Customized and built by ficeto [Team Darky Dev Crew]
  
  Based on supercurio’s latest voodoo sound 6, lagfix and color - thanks a ton, Mr. Supercurio


If you install on top of a kernel with Voodoo enabled already then you should be fine (and I don't believe it will disable it, but you can re-enable if it does). There are a handful of threads on the Darky forums that talk about some of the nuances to this, but here are a couple:


The Quick Start Guide - Recurrent Questions (The "Recommendations prior to flashing a ROM" section talks about lagfixes).
activate lagfix in 9.2?
What is a lagfix and how to enable or disable it (This one is kind of goofy but still helpful)