Performance -VPS Setup

I need some advice on my situaton at my host, and possibly some frame of reference as to what can/should be expected from a VPS setup like mine and what I can expect it to manage.

I have a site that sees some traffic of about 150k pageviews per day. On any given day, it peaks for roughly a timespan of 4 hours per day where there may be about 5 req/s.

I use a standard setup (LAMP) running mod_php in Apache, not fast cgi. I have a VPS on Virtuozzos Power Panel that has 1,5 GB RAM and really an unkonwn amount of CPU. I haven't been able to ascertain that information but probably could if I asked my host.

The problem is that during these hours it gets a bit slow from time to time. Running TOP shows sometimes a staggering amount of waiting processes i.e. the load is quite high (15 - 25).

So, I'm now really at a fork in the road where I either start looking into going with a different setup, say Nginx + PHP-FPM (FCGI) and try to see if that makes a difference. I'm not really an admin so I would be kind of lost on that. I could also start looking into my code to see if I can cache more or do smarter stuff etc.

However, before doing any of the above, I'd like to ask this crowd here if you think that I've sort of hit the roof on what can be expected from a VPS of the size I just told you about. That my situation is quite normal and that the real solution is actually just to upgrade my VPS. Is it?