gdi32.dll + unsafe code on 32+64Bit Windows

Question

Which target cpu should I use to compile a .NET 3.5 app (developed on a 32 Bit System) that contains an assembly which uses

 [DllImport("gdi32.dll")]
 static extern IntPtr CopyEnhMetaFile(IntPtr hemfSrc, IntPtr hNULL);

and which also has the option checked: "Allow unsafe code" because of the following code:

 ((byte*) dst.Scan0.ToPointer())[(dst.Stride*y) + x] =...

so that the app is able to run both on 32 and 64 Bit Windows ?

If the answer is "x86", does it mean that all referenced assemblies of the app have to be compiled with "x86" or just the "app.exe" ?