The answer, like in may things computing is, it depends....typically we will see only a 2-5 % performance increase in average fps.....but the increase in total system cost is less than that, then it is kind of hard to argue against the investment. In a addition usually where ya see an impact, it's on min fps more than average.
In the above link, we see a 4.5% increase in min fps and that will come at an extra cost of $44 for 8GB. On a $2k box, that's a 2% increase in cost or about a 2 to 1 return on investment. Some would argue that you won't notice the difference, then again if ya spending $2k, ya ain't likely to notice the $44
Here's a other investigation where the results were a bit more dramatic
http://www.anandtech.com/show/2792/12
22.3 % (SLI) increase in minimum frame rates w/ C6 instead of C8 in Far Cry 2
18% (single card) / 5% (SLI) increase in minimum frame rates w/ C6 instead of C8 in Dawn of War
15% (single card) / 5% (SLI) increase in minimum frame rates w/ C6 instead of C8 in World in Conflict
Here's what I am currently putting (All DDR3-1600) in my builds:
$50 CAS 9 Corsair Low Profile Vengeance 2 x 4GB
http://www.newegg.com/Product/Product.aspx?Item=N82E16820233196
$65 CAS 8 Corsair Low Profile Vengeance 2 x 4GB
http://www.newegg.com/Product/Product.aspx?Item=N82E16820233199
$95 Mushkin Low Profile Raddioactive or Redline
http://www.newegg.com/Product/Product.aspx?Item=N82E16820226221
http://www.newegg.com/Product/Product.aspx?Item=N82E16820226178