Gene,
If the lags come with greater usage, then it is potentially the limits of peak network through-put; if you will, the diameter/size of the pipe.
Based on a quick peek, it appears you are running a 10/100 switch. A layer 2 (or 3) switch should give you better peak/burst performance, but that is constrained by how many machines already have Gigabit (1000) network adapters to take advantage of the potential greater through-put.
For a practice of 20 machines, we recently installed
http://www.amazon.com/gp/product/B00358MP02, Netgear GS724T-300.
One of the things that you can also consider is adding a second Gig network adapter, and 'balance' the traffic over both adapters, or take the second adapter and plug it directly into the wireless router.
