Then any USB 2.0 HDD will be fast, as USB 2.0 limits you to 35 to 40 MB/s read or write speeds, thus even the slowest currently available HDD will work perfectly fine with that.
Even if you get a Firewire 800 HDD (with an USB port too), most HDDs will be fine again, as FW 800 is the limiting factor (65 to 75 MB/s).