Should i upgrade my ubuntu 10.4 to the latest 11.4 ??

I would recommend you that you wait a little longer, because ubuntu 11.10 will be soon finished (https://wiki.ubuntu.com/OneiricReleaseSchedule). If you are really impatient, you should give it a try. Ubuntu 11.04 is now a lot more stable than it was on the launch date. And if you aren't 100% sure that you like ubuntu 11.04 and you have good enough computer you can also try it in virtualbox.

Thanks friend ......i could not wait and already have installed Ubuntu 11.4, but i had installed it so that i could install bisigi themes but sadly i could not completely install it, the login screen and the button did not change.....only the desktop backgound and icons did change .....need help .....do you how to fix it ???

I don't understand you completely what are you trying to say. If I understood you right: when you install a theme, the login screen won't change. You have to tweak it manually if you don't like default one. To change login screen's background you can use a software, called Ubuntu Tweak (http://ubuntu-tweak.com/) or you can take a look at this tutorial (http://www.webupd8.org/2011/05/change-gdm-theme-background-in-ubuntu.html).

right the login screen and the login buttons (i dono what they are called ) where we get to choose user to log in, these both dont change, i want them to be installed automatically when i install the theme......

I have tried GDM but it can only change the login screen what about the login button and other greeter features.....how do i install them automatically ......??

Also the bisigi themes are supposed to be installable easily and quiet compataible with ubuntu 11.4 still why the problem is occuring ??? am i missing somethin ???

Also note : i cannot install tar.gz theme files downloaded from gnome-look.org says invaild ...... why ??

thanks ..

if u get to know how to change GDM theme and customize ubuntu please do tell me....thanks ... :)