Multicasting?

Multicasting is a possibility, but not the way you are thinking:

A big pipe at 100Mbps Full duplex is gonna run you $300-$1000/mo
A typical server is lucky to spew out as much as 60Mbps (Thats doing a disk dump)

Now, 5000 users * 4 pkts/s = 20000 pkts/s, at 1520 bytes/packet == 243Mbps
Theoretical Max packets of that connection = 100Mbps / (1520 * 8 bits/ packet) = 8223 pkt/s
Clearly you are talking multiple pipes (or an OC12 - 622Mbps) and a server farm

So your configuration for this is basically
Node A         Node B          Node C
   |                   |                  |
   +--------------+--------------+
                       |||
(at least 3 100Mbps lines or more likely an OC12)
                       |||
                CPE Fiber Demarc
                        |
                  (The 'Net)
                        |
Client1 Client2 ..................................................Client 5000

Ok, how many nodes?
Well if each one is doing processing, you can max out the PCI bus ~ 133Mbps, using Gigabit ethernet you should be
able to blast out about the same.  (Kinda depressing, really).  Better bus architectures *could* let you spew up to 3-400 Mbps, but not using regular (1500byte) packet sizes.  So more servers.
Max load ~243 Mbps, so just an off the cuff figure of 4 servers should handle the load nicely.
You can get away with fewer servers if two of them are not doing any processing:
                Broadcaster
                       |
   +--------------+--------------+ (Multicast source on 224.0.0.1)
   |                   |                  |
Node A         Node B          Node C  (Relays listen on 224.0.0.1 and rebroadcast to TCP connected clients)
   |                   |                  |
   +--------------+--------------+(Regular TCP network)
                       |||

Well, theres your blueprint

Cheers,
-Bill

Thank you Bill.

You correctly consider all the packets of max size but it is not so in my condition.

Packet dimension is about 190 bytes so 4*190*8 is 6 Kbits/s per client that is about 1/8 of what you calculate.

However, also considering that, I need to understand pros and cons of multicasting solution and if this way is not right the better other one.

Thank you again.