You should be able to upgrade, where did the Standard key come from? Dell?
http://www.microsoft.com/windowsserver2008/en/us/foundation.aspxIf you are a small business running Windows Server Foundation and experience the above mentioned conditions we recommend that you upgrade to Windows Server 2008 R2 Standard. You can obtain a copy of Windows Server 2008 R2 Standard from your reseller.
http://www.petri.co.il/introduction-to-windows-server-2008-foundation.htmUpgrading Windows Server 2008 Foundation
Windows Server 2008 Foundation supports the following upgrade scenario:
•“In-place” upgrade of Windows Server 2008 Foundation to Windows Server 2008 Standard.
Windows Server 2008 Foundation does not support the following upgrade scenario:
•“In-place” upgrade of Windows Server 2008 Foundation to Windows Server 2008 Standard Core.