For most developers, the choice will be Windows 7 64-bit. The last part is easy: SharePoint 2010 only installs on a 65-bit o/s ...so either Windows 7 or Windows Server 2008 ...and yes, you do need to use 64-bit SQL Server as well.
I personally have Windows Server 2008 R2 running Hyper-V on my Dell laptop with 4GB of memory. Under Hyper-V, I run Windows 7 VHDs for SharePoint 2010 development and Windows Server 2008 for SharePoint 2007 (and general ASP.NET) development.
I use 3GB of virtual RAM for SharePoint 2010/Windows 7 and sometimes think I should buy more RAM. 1.5-2.0GB is fine for SharPoint 2007/Windows Server.
To do SharePoint 2010 development properly, Visual Studio 2010 is also required. The combination of Visual Studio 2010, SQL Server 2008 and SharePoint 2010 in a single VHD means the more memory you can afford the better.
For details on how to create a SharePoint 2010/Windows 7 developement environment, checkout the following MSDN article: http://msdn.microsoft.com/en-us/library/ee554869(office.14).aspx
Two things to remember are: in Control Panel > Programs > Windows components, you need to add the non-HTTP support for WCF and you may need to uninstall/re-install ADO.NET Data Services if BCS (Business Connectivity Servicess) and/or REST, respectively are working on your machine.