I have several devices connected to my Virgin router - which is a dual band system. But although both laptops have 802.11b/g/n wireless cards, only one of them recognises the 5 GHz signal. The printer, both Ethernet bridges and the Philips Streamium all use 2.4 GHz.
So my old Dell Vostro 3300 provides a download speed of 109 Mbps on 5 GHz, yet the newer HP TouchSmart only achieves 19 Mbps on 2.4 GHz, although upload speeds on both are around the same at about 6 Mbps.
I know that the TouchSmart is rather a limited device (and it's only used as a backup or for the odd bit of surfing), but whether the slower downstream speed is due to the wireless card or the overall low spec. of the computer, I do not know. I guess we've now reached the stage that 'internet speed' is often more limited by the device than the infrastructure?