Ed Brill is making hay with Microsoft’s system recommendations for Exchange 2007 beta 2. (Don’t miss the comments, especially the ones pointing out that IBM doesn’t even publish per-user resource guidelines for their own products– good thing, because if they did Workplace wouldn’t look too spiffy!)
Anyway, Ed’s article misses the point: the recommendations are for servers with “many users with large, frequently used mailboxes”. If you don’t have many users, or they don’t have very large (>1GB) mailboxes, or the mailboxes aren’t frequently accessed, you can get by with much less RAM.
Remember, the point of adding RAM is to reduce the number of I/O operations per second (IOPS) that you need to handle a given user load. Large mailboxes and frequent accesses mean more IOPS. More IOPS means more disk spindles, which means lots more money. Gigabytes of RAM are cheap compared to SAN disks; right now, Exchange 2003 servers scale out by adding more spindles to get more IOPS. With Exchange 2007, you have a choice: add IOPS by adding disks or reduce the number of required IOPS for the same user load by adding RAM for caching. You get to choose according to your needs– part of Microsoft’s promise to provide more administrator choice and control in Exchange 2007. (Take a look at this post for more detail on disk I/O tradeoffs in Exchange 2007.)
Ed’s pricing example is a little disingenuous too, because he doesn’t specify how many Notes users his hypothetical 6200-user Dell configuration could host, and he ignores storage costs altogether. I’ll be happy to put together a reasonable configuration for N Exchange users and cost it out if you’ll do the same for Domino. (I’ve made this offer before, and Ed’s ignored it– wonder why?)