Need Suggestions / Opinions about Load Balancing + some technical questions!

Your bottleneck is more than likely apache and its memory hunger.

A bigger server will only move your bottleneck somewhere else, likely onto the hard drives and IO speeds

If your doing load balancing then a server with decent amount of ram (probably 4gb should cut it) will be the needed, I would also recommend using a pair of LB servers in IP failover configuration which will also give you HA capabilities. for software i would recommend Pound, HA-Proxy or even nginx.

if your not going for HA capabailities but purely maximum capacity then it would be much much better to make sure your hardware is as close as possible on the network so as not to add any other extra latency which can cause other performance problems.

before you shell out extra costs you may find that switching your webserver to something more memory efficient may buy you a lot more time (and savings), Litespeed is something that can be used as a direct Apache replacement (but costs) and Nginx is free but would probably require some modification of your setup.

Other considerations would be caching, the use of Varnish etc