Every environment is different but I tend to agree with ledow.... Based on what you have stated, I would think your IT dept is sufficient in size.
I work at a 400 user company (wholesale/retail) with an IT staff of 4: a developer, an ERP help desk person, a IT director who also manages the ERP system, and myself, the sys admin who handles everything else. We have 30 branch locations a commercial SAN, about 16 virtual servers and 8 or 9 physical ones.
What helps hugely in my case is that the bulk of my users are on thin clients. I've only got one PC at each remote site and maybe 100 at HQ. Everyone else is using thin clients connecting to our 4 RDS server farm. Even so, that leaves 130 or so windows 7 laptops & desktops but I typically only get a handful of calls a week from people with PC problems. As for my servers, I'm actually shocked at how stable and maintenance free they have been - All Dell & Hyper-V. Server 2008R2/Exchange 2010.
I do all the networking, server & SAN management, and desktop support myself, and I frequently wish I had more big projects because I can get quite bored when things are running smoothly. I'm not even a l33t coding admin. Just a competent old-school point and click one. Also I tend do do things cheaply which, ironically, also reduces complexity (in my case). I stay away from the commercial high end backup, system/network management, or security solutions, most of which are geared to enterprises much larger than ours. I stick to free and/or cheap, and for the most part and it all runs very smoothly. The first few years of setup and clean up were a lot of work but these days my network practically runs itself.
What I don't have is an overbearing management structure with unrealistic expectations and requirements, so that also helps a lot. What I do have is 400 people who generally think I'm the best thing since sliced bread and a lot of free time at work.