This is a question involving too many variable. In the past I have seen servers with slower speed ram easily out perform servers with much faster ram, generally it is the reverse. "The R710/R910 would have about a third less CPU performance and around 50 gigs of RAM, thou it would be 1333Mhz." CPU speed trumps ram speed generally, especially 33% difference, even considering web servers are primarily read intensive. Not sure of how much disk space you need, but if it is a reasonable amount I would look into SSD drives for extreme disk speed. If not SSDs, and if you have the resources, raid 10 is pretty fast. 
Hi and greets from Finland!  I'm suppose to accuire new hardware, and I'm not sure what's the most efficient way to go.  I have two options at hand: the R900 or R710/R910 Poweredge.  I'm just wondering, how much does RAM speed (not the amount but speed) affect a server that should host a website? The website/s is/are quite heavy and does at least put a toll on the viewers side of performance, but since the R900 supports only 667Mhz RAM, is it really worth to at least double the budget and go for R710/910 that supports 1333Mhz RAM? Does the RAM speed affect a server that much?  Overall the R900 would have a total of four 2,94Ghz quad-cores and around 100 gigs of RAM, 15k HDD's The R710/R910 would have about a third less CPU performance and around 50 gigs of RAM, thou it would be 1333Mhz.   What do you folks think?
RAM speeds impact on website/server