OK, and so i am using nginx + php fastcgi, I have to show site visitors an custom error page stating that the we're going through technical problems, how do i do this ?

Must I install nginx on 80 as proxy to a different nginx server hearing 8080 and appearance it's status with HttpHealthcheck Module, or it is possible to better solution ?

You have to an third party addon to check on your after sales server, take a look: http://wiki.nginx.org/HttpHealthcheckModule

Once the after sales is lower, nginx answer an 502 error, you are able to set a custom page for this.

error_page   500 502 503 504  /50x.html;
location = /50x.html {
    root   /usr/share/nginx/html;
}

New answer :)

I'd use haproxy(http://haproxy.1wt.eu/) for your.

haproxy as the frontend listening on port 80, and nginx+php-fpm as after sales

setting up HAProxy: (verify your configurations and edit the scripts)

make TARGET=linux26 CPU=x86_64
cp haproxy /usr/sbin/haproxy
wget http://layer1.rack911.com/haproxy/haproxy-standard.cfg -O /etc/haproxy.cfg
wget http://layer1.rack911.com/haproxy/haproxy.init -O /etc/init.d/haproxy
chmod +x /etc/init.d/haproxy

haproxy.cfg (I didnĀ“t range from the global/default periods)

frontend webserver-80
        bind <ip>:80
        option forwardfor
        option http-server-close
        default_backend backend-nginx

backend backend-nginx
        #balance roundrobin
        balance source
        option httpchk GET /fpm_ping
        server srv1 <ip>:<port> weight 1 check
        errorfile    503        /etc/errors/503_noserver.txt

Check hatop util (http://code.google.com/p/hatop/) or have an stats session to haproxy.cfg so that you can check up on haproxy status...

listen stats :<port>
        balance
        mode http
        stats enable
        stats auth admin:admin
        stats uri /
#       option   httplog

Configure php-fpm to reply the ping demands (or change it out with other check)

On /etc/nginx/nginx.conf

location /fpm_ping {
    access_log     off;
    allow          <ipaddr/cidr>;
    allow          127.0.0.1;
    deny           all;
    root           html;
    fastcgi_pass   127.0.0.1:9000;
    fastcgi_index  index.php;
    fastcgi_param  SCRIPT_FILENAME  /usr/share/nginx/html$fastcgi_script_name;
    include        fastcgi_params;
}

On /etc/php-fpm.d/www.conf include/uncomment

ping.path = /fpm_ping
ping.response = pong