What applications are people seeing real performance gains with 4Gb?

Or more to the point, what applications will benefit from more than 4GB?

The principal advantage of 2x2GB kits (so far) is that you're not using all 4 slots. I don't see a cost or performance advantage, at least not yet. These kits are somewhat expensive (on a per-MB basis). They are fast, but it's been documented that 4x1GB performance is at least as good. Besides DDR3 is around the corner, so do I want to invest the potential capacity for 8GB of DDR2?

Am I missing something?