SBS 2011 and ESXi

Should I get SAS disks (if so which ones) or enterprise SATA disks.

If you need 10rpm or more then go SAS. If 7200 is enough and judging by the amount of users it will be then go wirh SATA.
Another note, I have 6 servers with SAS drives (HP DL 380 G5 and G6) and around 10 with SATA drives. In last 4 years dead ratio SAS:SATA is 0:3

I was a bit shocked when I read the system requirements for SBS 2011: http://www.microsoft.com/sbs/en/us/syst ... ments.aspx What's so special about SBS 2011 that it needs a quad-core processor? Why wouldn't a dual-core suffice?

It's a bulls***. There is nothing special. I'm guessing it's recommended because you can have "whole package" running on one maschine.

Ideally I would've liked to have just migrated our SBS 2003 installation onto a new server. However as we have the OEM version this isn't possible. So I'd like to get a Retail or Volume License version of SBS 2011 (which do you recommend?)

It is, well at least it was. Also depends who you get on the phone @ Microsoft. The EOM version is assigned to motherboard. I have 50/50 success rate :) Once I was able to transfer license from one machine to another because I had to exchange motherboard. After that I had to reactivate Windows once I was able to do that, got new activation code from MS. Another time I wasn't so lucky.

Also I was thinking using ESXi would be a better way of doing things in case we wish to get the Premium Add-on down the track and also to make it easier to move to a new server if the current server fails.

That's for sure. I have 5 ESXi servers running with 8 OS on each. Migrating machines is easy and fast.

I'd like to be able to separate hardware upgrades from OS upgrades when it comes to the server. As an SBS migration is a time consuming task.Any advice on how to migrate from SBS 2003 to 2011 such as good sites to look at?

Well depends what you have running there.

Also how much downtime will this lead to?

Also depends what you have running. AD, file server, print server, Exchange, SharePoint you can migrate really with really short or no downtime at all.