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 both a Registered Nurse and a Microsoft SQL Server MVP 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. Experienced in systems integration and workflow analysis, John is passionate about combining his IT experience with his nursing background to solve difficult problems in healthcare. He sees opportunities in using business intelligence to satisfy healthcare meaningful use requirements and improve patient outcomes. Contributing author to
SQL Server MVP Deep Dives and
SQL Server MVP Deep Dives Volume 2.