server usage/performance - what to expect from an entry level rootie

I know this is totally off topic, but I thought its still a very good place to ask:

I am having some problems with my root server but all it does is providing email for < 10 users and hosting ca. 10 wordpress installations + 1 wpmu so I am somewhat at a loss to find out where the problem lies. These are all extremely low traffic sites, as the wordpress isntallations are just personal websites of friends and family and the wpmu install is my testbed.

Can someone offering serious hosting tell me if what I ask is too much from a 1GB RAM dual AMD (forgot the exact spec of the proc) processor server?

I just want to make sure I am not asking to much from this server...
Does anyone here on the forum offer a server-optimization-or-analysis service?

I know some of you are into hosting, so maybe you can show some figures how many clients you cram onto a server without problems...