Windows 2008 R2 SP1 64bit is your best option, it's the most recent.
As to Win7 vs 2008: one is a desktop optimzied OS, the other a server optimized OS... should be a no-brainer.
Curious: Are the Small Business editions not included because they are known to not work - or - just not yet tested ?
*Windows Small Business Server 2011 Essentials $545 Includes Small Business Server 2011 Essentials. Price also includes a 25 user account limit and CALs are not required.
*Windows Small Business Server 2011 Standard $1096 Includes Small Business Server 2011 Standard Edition. 75 users Price also includes 5-pack of Small Business Server 2011 CAL Suite (User or Device, chosen after purchase)
*Windows Small Business Server 2011 Premium Add-on $1604 Includes Small Business Server 2011 Premium Add-on Edition. Price also includes 5- pack of Small Business Server 2011 Premium Add-on CAL Suite for Users or Devices (User or Device, chosen after purchase)
The SBS editions include server software that is typically bad to put on a database server (like Exchange) or has has had known compatibility problems (like SharePoint).
SBS typically is also meant to be the file/print server and AD. All roles that tend to significantly increase the risk of badly affecting FMS (and not just performance).
Thanks - I would not have know that otherwise.
And 2008 R2 64 Bit does not have to be that expensive. I regually find them for $200 or less on eBay, never used, with money back guarantees In fact, there are several there now, one from a "top rated seller" for $185. Once you start using it, it will install Sp1 (no cost).
There is an additional expanse for CAL's, however you can find them on eBay also.
As a bit of more information, trying to install older versions of FMS on newer SBS versions is getting harder and more unreliable. FMS Console needs Java and the older versions need special tweeking to work at all.
I make recommendations 2008 Standard R2 ( which is only 64 bit ) but a client supplied a server box for me lately with SBS Essentials 2011. It is lightweight, no exchange or sharepoint, has got a good backup routine.
But still has a conflict somewhere not with server itself but the closing of FMS (as I said an old version), such that a reboot of the server for any reason causes damaged files. This is after issuing 6 times net stop "FileMaker Server" before rebooting. Nice :-<
Stay away from SBS and you will save yourself some grief.