I set it to 32 veins per chunk to make sure I could find the ore when mining; that's not the final values.
I had no problem running without the nether-generating code with 1GB, but even with 2GB I ran out of heap when I enabled the nether code.
I'm using an IWorldGenerator to generate veins of my custom ores, and it works just fine in the Overworld, but when I use the same code in the Nether it blows up with OutOfMemory errors.
Is there anything in particular to be aware of when generating in areas other than the surface?