If you have a few WiFi based devices/computers it would probably be better to upgrade the router, (the Netgear WNR3500L I mention above or similar), that way you'll get an all-round higher throughput, assuming of course that you upgrade the WiFi devices to 802.11n also, plus by plugging in the USB HDD, an easy NAS solution.
The difference in price between the Addonics NAS Adapter V2 and the Netgear WNR3500L is ~EU$70, (EU$41 vs EU$110 ex. any shipping and ripoff). Then your limiting factor will be the 802.11b/g in the rest of the WLAN.
If you only have one WiFi device then perhaps a pair of HomePlug
units, (at least the 85Mbps versions), might be suitable to increase throughput between it and the router, (assuming suitable electrical house wiring of course), the price of them has been progressively dropping and the 85Mbps units will be cheaper than the HomePlug AV 200Mbps versions.
Even if the routers USB port is v1.1 you should be able to get at least 1MBps throughput via the HomePlug units.
Of course, for the cost of these adapters, (and a bit of exercise), you could also buy a fair amount of CAT5 cable and get some real speed