The bottleneck in the path (based on choosing the switch with the lowest model number, on the principle that bigger model numbers means mroe fastar1!1) is probably a Dell PowerConnect 3324 with one SX SFP, one LX SFP and all the 100M ports pretty much idle. There are no increasing error counters on transmit or receive on any of the devices in the path.
Is it reasonable to expect a Dell 3324 to be able to switch more than ~500M of internet-sized packets?
Specs: http://www.dell.com/downloads/global/products/pwcnt/en/pwcnt_33xx_specs.pdf
Packets: 6.5Mps Capacity: 8.8Gps
smallest ethernet packet size: 64byte (1GB * 1024*1024*1024/ 8) / 64 = 2097152 = 2Mps
Ups, one failure, we have full duplex: (2GB * 1024*1024*1024/ 8) / 64 = 4194304 = 4Mps also enough. bye, Ingo