If we're talking about a 1,1, ram is quite expensive. If it's a 4,1, go to 16GB and enjoy, as you can get that amount of ram around $100 on the 4,1. If you're working on large files, you can use it. If you like it to maintain many history states + thumbnails and other cached elements with large files, you can use large quantities of ram. You can see gains past 16GB too, but at that point it starts to really depend on your workflow. If you're only running photoshop, even with big files that tends to grant decent performance.