This code should give you the "bitness" of Excel.

Microsoft.Office.Interop.Excel.ApplicationClass app = new Microsoft.Office.Interop.Excel.ApplicationClass();
if (System.Runtime.InteropServices.Marshal.SizeOf(app.HinstancePtr) == 8)
{
    // excel 64-bit
}
else
{
    // excel 32-bit
}

EDIT: here is another version that should work for previous versions of Excel as well. Just pass an ApplicationClass reference to it:

    public static ExcelVersion GetExcelVersion(object applicationClass)
    {
        if (applicationClass == null)
            throw new ArgumentNullException("applicationClass");

        PropertyInfo property = applicationClass.GetType().GetProperty("HinstancePtr", BindingFlags.Instance | BindingFlags.Public);
        if (property == null)
            return ExcelVersion.Excel;

        return (System.Runtime.InteropServices.Marshal.SizeOf(property.GetValue(applicationClass, null)) == 8) ? ExcelVersion.Excel2010_64 : ExcelVersion.Excel2010_32;
    }

    public enum ExcelVersion
    {
        Excel, // before 2010, so 32 bits
        Excel2010_32,
        Excel2010_64
    }