What's the best practice re: the GC for a performance-sensitive app like a 3D XNA game?

Littleguru wrote:
- Why isn't it possible to force the GC to do a collect. I mean GC.Collect() just signalizes that a collect is required, but the GC could decide to do not. Why has this been implemented in that way?

If this is indeed true, can't we have a GC.Collect(bool meanIt); ?