display VF component based on variable value

This question has been asked previously here :

Loading a custom component into a Visualforce page dynamically

So this might (or) might not help you. I have introduced if loops but not sure if there is another way.

I am not aware of any way to dynamically construct an apex tag.

I took the approach of controlling the component to display in the controller where we have much control to do what we want.

Thanks to @sfdcfox. Based on this question I constructed the answer with a bit twist to accommodate what you are looking for:

https://developer.salesforce.com/forums?id=906F000000092Y5IAI

Page:

<apex:page controller="comp_test_controller">
 <apex:dynamicComponent componentValue="{!headerComponent }"/>
</apex:page>

Controller :

public class comp_test_controller {

    public transient ApexPages.Component headerComponent { get; set; }

    public comp_test_controller(){
        Type t = Type.forName('MyComponent');
        if(t != null) {
            for( ApexPages.Component cust : ((CustomComponentManager)t.newInstance()).newComponent()){          
                //In your case you will compare cust with the Component_Name__c value.
                if(string.valueof(cust) == 'Component.sitelogin'){
                    this.headerComponent = cust;
                }
            }
        }
    }
}

Class to support type.forname:

Previously type.forname('') used to support components, but it supports only apex class instances now.

https://www.salesforce.com/us/developer/docs/apexcode/Content/apex_methods_system_type.htm

    public class MyComponent implements CustomComponentManager {
    public List<ApexPages.Component> newComponent() {
        List<ApexPages.Component> comp_list = new List<ApexPages.Component>();
        comp_list.add(new Component.c.SiteLogin());
    return comp_list;
    }
    }

Interface:

global interface CustomComponentManager {
      List<ApexPages.Component> newComponent();
    }