Which version of Sql Server 2008 do you use with your Enterprise Moss installation and why?
More specifically, do you actually need all that the Enterprise Sql Server version provides or is the standard version more than capable to provide service for a company of 1000+ users. What about 5000 users?
As an added note, we will be running the entire installation in an esx farm with a SAN data source.
Rather than counting the number of users, I'd ask a few other questions.
Does it need to be accessed by end users 24/7? If so, you need Enterprise Edition to get online index rebuilds. If you can get maintenance windows after hours or on the weekends, then you can get by with Standard Edition, which takes indexes offline while rebuilding them.
Have you tested your IO throughput? You said you're using ESX - if it's v3.5 or earlier, or if you're not using the top license tier for v4, then you don't get multipathing capabilities. Your throughput is confined to just one host bus adapter per array. If these terms make your eyes bleed, here's the short story - you might be bottlenecked at just 200-400MB/sec, which can be saturated with a dozen SATA drives. No matter what kind of SAN you hook up, it won't matter if you can only get one HBA of throughput at a time. Standard versus Enterprise will be the least of your problems.
How do you plan to do disaster recovery? If you're thinking about using log shipping or database mirroring, then look into Enterprise's capabilities. If you're doing mirroring, then you get some slick features around corrupt page recovery by using the mirror's SAN instead of the primary.
Are you storing sensitive data or documents in SharePoint? If so, you should consider Enterprise to take advantage of Transparent Data Encryption. That encrypts the data at rest, when it's sitting on the SAN, thereby preventing someone from grabbing a SAN snapshot or a copy of your backup tapes and restoring an entire copy of your database when you're not looking.
Don't worry about compression. Enterprise adds the ability to do data compression, but don't let that sway your decision. Your SharePoint users are probably going to be storing a ton of binary documents, which don't compress very well anyway.
For a customer we chose a Sql Server 2008 Enterprise Ed. 2 node cluster for high availibility, not speed. The design is a conservative straight out of the Microsoft playbook 5 node farm with 3 front end MOSS nodes on Hyper-V and a Dell Equallogic iSCSI SAN.
The SQL cluster was not virtualized since at design time (mid 2008) the virtualized SQL 2008 cluster configuration was not supported by Microsoft.
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If you are starting with around 1000 users and might end up with 5000+ users, I'd say it's a typical scale-up project, and I would defo go with enterprise edition. I would probably go with a clustered backend right from the start.
That said, it all depends on the traffic pattern of those 1000-500 users and the service level you wish to deliver. It's not impossible to switch from standard to enterprise later either, but it will cost you some downtime, which might or might not be an option.