Corsair has run a long series of tests on their 1600MHz Corsair Dominator TR3X3G1600C8D series, comparing the performance between 3GB and 6GB on the new triple channel X58 based ASUS P6T Deluxe motherboard, an Intel Core i7-965 Extreme Edition, a pair of 1GB Nvidia GeForce 280 GTX in SLI and 2 Seagate Barracuda 7200.10 320GB in RAID 0.  (So, you know, a system just like everyone has) 

Does the extra 3GB of RAM have an effect?  Read through their PDF to see where you can expect to notice gains and where you are less likely to.