SQL Server 2008 R2 on Windows 7 x64 Professional - what are the downsides? I need to minimize the cost of the platform, that will run SQL Server 2008 R2 Standard Edition. I have a special licensed version of this SQL server to work with like 20 users (it's an 1C special conditions licensing to be used with 1C products only) with costs not too much. I don't want to buy Windows Server license because with this number of clients (20) I need to buy Enterprise version, which is about $5000.


SQL Server 2008 R2 Standard supports as I can see Windows 7 Professional.  
Windows 7 Professional x64 supports up to 192Gb of memory so that's prettty enough.  
20 inbound connections limit does not apply to TCP-IP connections as I know.


What are the downsides of using Windows 7 x64 Professional as a host OS for SQL Server 2008 R2?
Your going to run into occasional memory and thread handling issues.  I run 2008 r2 Developer (which is basically enterprise) on Win7 Prof (64x) on a development machine* for writing code.  I do not know the exact reason but SQL acts funky sometimes and you need to either restart service or box.  Ex. No queries will be running and cpu will be pinned at 50% with SQL proc that if you check with ProcExplorer isn't doing anything. If this is something you can live with doing every so often then maybe you can get away with it.  On the other hand if you need high up-time then spend the money on server edition. 

*I run like this because Visual Studio runs like crap on server and I like a portable compact dev setup when I'm on site.