Do I Need Separate Apache & Mysql Servers Etc

here's my current setup has my stuff running on 2 separate "self contained" servers (eg; each runs their own apache/php/mysql):

Main site/server:
content: mostly static content (no mysql, very little php). currently has about 4tb/m traffic. in the summer it could push up to 6tb/m
hardware: P4 2.8ghz. 1gb ram. this server has no problem handling the load. only problem is bandwidth (i have to get it off the current host)

Forums site/server:
content: running vBulletin. currently 400-500 peak (probably jump to 800 peak this summer) users active per 15 mins.
hardware: 64bit dual Opteron 242. 4gb ram. it's absolutely griding that to a halt at peak times. it acts like 4gb memery isn't enough (it will run fine then eat through most of the 4gb. grind to a halt, then recover) personally i think it was setup/configured wrong but i've had multiple people look at it and nobody can find anything wrong in the apache/mysql settings.

What I want to know:
what type of server setup should I start migrating to? should I keep both parts of the site separate? eg; main site on one server, forums on another server(s)?

what i've sorta been looking into is 3-server setup.
(server1) main apache/php server. probably on unmetered (honestly don't think i'll find anything else that offers high enough traffic). run the main site and the vB php from here
(server2) dedicated mysql 'read only' server (server3) dedicated mysql 'write only' server.

and have mysql read/write synced and have all 3 servers networked directly together. i have a friend running a single mysql driven site using this setup and it works really well for him.

is this overkill for me? should the current dual Opteron be able to handle the forums and i just need to hire someone smarter, or is there some other setup that would work better for me? i'm tight so i want to go cheap as possible, but I also realize i need room for summer traffic expansion that always hits us.