Tweaktown posted 3GB vs. 6GB Memory Performance Analysis on X58. Here's a taster:

One of the biggest advantages when moving from 2GB of memory to 4GB of memory was that your overall Vista experience felt smoother and snappier. But moving from 3GB to 6GB doesn't really give us this effect and with 3GB of memory everything is still running very nicely. While for most people they thought that going from 2GB to 4GB of memory gave the best performance increase, it was more so moving from 2GB to 3GB+ that helped the most. Does this mean that 6GB kits are pointless? No! - For people who multi-task heavily, the extra memory is extremely handy. Also, start throwing programs like Photoshop and other graphics rendering / creating programs into it and the extra memory is not really wanted; rather, it's needed.

From a gaming perspective, though, even at the ultra intensive 2560 x 1600 resolution the extra memory does very little for extra performance and while your games might load ever so slightly faster, for people on a tight or strict budget the increase doesn't warrant the extra cost that is carried with moving to a 6GB kit. Depending on who you are will decide on how much memory you get. If you have the money or just want the best you're going to end up with 6GB. If you multi-task a lot and use your computer for work that runs memory intensive programs, then you will no doubt end up with 6GB or even 12GB. If you're on a budget or would rather spend the money you save by buying a 3GB kit over a 6GB one in order to upgrade your graphics card, you will end up with 3GB of memory and still be extremely happy.