My server load crossed 150 ?

Last week my visitors were increased and memory usage crossed 90% and server loads jumps from 0.90 to 150+ and sever gets down

at that time total ram for server was 2 gb

now i increased the ram and now the total ram is 4gb

im seeing right now there are max ever visitors on the site and still sever load is below 0.50

i want to know that if i there is anyfurthure overloading in server and server gets down should i go for increasing ram , how much max ram will be ok

And right now server memory usage is reaching 75% so should i increase ram or add 100mb switch port.

im on linux server

Thanks