The official list of supported operating systems, service packs, and numbers of virtual processors can be found here: http://support.microsoft.com/kb/954958/en-us. If you are willing to venture into unsupported territory, you might find some surprises. I have both 32 and 64 bit Windows 2003 R2 Service Pack 2 virtual machines running with four virtual processors. The same is true of Vista Service Pack 1.
Also of interest is what happens with PAE. Hyper-V requires that hardware Data Execution Prevention (DEP) be enabled in the BIOS. Intel calls this hardware feature the execute disable (XD) bit. AMD calls it no-execute page protection (NX). When DEP is enabled, Windows automatically enables Physical Address Extension (PAE) without having /PAE in the boot.ini. Because of this, your 32 bit guests will work with large ram as the screen captures show. For more information, see http://support.microsoft.com/kb/875352.
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About John Paul Cook
John Paul Cook is a Technology Solution Professional for Microsoft's data platform and works out of Microsoft's Houston office. Prior to joining Microsoft, he was a Microsoft SQL Server MVP. He is experienced in Microsoft SQL Server and Oracle database application design, development, and implementation. He has spoken at many conferences including Microsoft TechEd and the SQL PASS Summit. He has worked in oil and gas, financial, manufacturing, and healthcare industries. John is also a Registered Nurse who graduated from Vanderbilt University with a Master of Science in Nursing Informatics and is an active member of the Sigma Theta Tau nursing honor society. He volunteers as a nurse at safety net clinics. Contributing author to SQL Server MVP Deep Dives
and SQL Server MVP Deep Dives Volume 2