Dynamic Login field on site_header

I'm trying to create a login field on top of my site header next to my banner, as shown bellow

__________________________
|-----Banner------|-----DyLogin---|
--------------------------------------|
| N----|........Content---------------|
| A----|------------------------------|
| V----|------------------------------|
| -------------------------------------|
|________copyright__________|

First i put a simple Form into site_header, which worked very well, but then problem is when user is loged in, it still shows login field, what i want is some sort of script to check if user is loged on, if yes show welcome, $username you have $pm messages, click here to lo out and if not show the form

i tryed different thing includes,
puting $insert[login_dologin]
puting <? echo GetTemplate("login_dologin"); ?>
into Login section of my header

but none of them worked, i'm really stuck and dont know what to do, please help me

Thank you
esi.