Interesting, however with my craptastic setup I get minimal issues with heavy usage.
I am back with Dad for a few months now, and I have three machines doing something (one workstation, two lappies), my Dad's PC and brother's PC. All are generally active to some degree, whether it be streaming content, my WoW playage, or one of my myriad of vms/live systems going out and grabbing updates. We have the Uber crappy Motorola handout that locks every now and then, a Linsys as the main router, and my Netgear that adds wifi for me to use. All are using the built in onboard NICs. Nothing fantastic, but it works.
I have on occasion hosted LAN parties and have had zero issues playing CSS, WoW, Quake 4, UT 2004 where we all gang up on some lamers that need pownage No issues.
I have real reservations as to the real value of going and getting nice 3COM or Intel rackmount routers and switches to add at max a few percent of performance. Granted, I will always reccomend getting the better component, such as spending the money on a nice 3COM or Intel NIC, but when it comes to the REAL networking gear, there is little value.
Of course, this is my opinion based off what I have sold companies and have seen as far as results. $2000 or so to rig up a sweeet LAN is fine and dandy, but hardly works better than just using good NICs in the first place, since that is where the bulk of the work is being done -- the NIC itself and in the driver.
Of course, this is MHO, and YMMV