Hmps, seriouisly, you look at the probvlems from the totally wrong side. THere is no need yet to go to multiple servers.
Windows server 2008 8GB RAM 1MBPS band
Ok, lets ignore the low memory for a moment.
1mbit bandwidth are pathetic. Point. 1500 users in parallel overload the bandwidth. I am sure when checing you will find out that your CPU and Memor and disc are not the problem, but your bandwidth is.
Putting a number of load balanced web servers behind the 1mbit line wont change that.
Upgrading the line means your server can serve more without needing more hardware.
Get proper bandwidth and things are good. 1mbit is really low these days. Actually it ALWAYS was low for a 1500 parallel users server. Never worked. A server today shoul be on a minimum on a 8mbit link, profesionally more on a 34+ mbit link. Hosting servers wont offer you technically less than 100mbit.
If it is a money issue, realize you are totally fixing the wrong thing. YOu need more bandwidth.
Oh, and voted to close - serverfault.com