The default version reported should match the level of compatibility the developer's think they're at. Although having a changable option sounds nice... both basic (select NT4, 2000, XP, etc from a list) and advanced (dig into the actual numbers). A little like SETVER (MS-DOS) and Compatibility Mode (hidden in Win2k; enabled in WinXP), you should have per-application settings as well.
StyleXP uses Windows XP's own visual styles, it does not add any extra skinning functionality (WindowBlinds does, though). In order for that to work on ReactOS, we need to implement a visual styles (uxtheme.dll) that is compatible with Windows XP/Server 2003 first.