I would like to point out that the combination of software timing, USB, and Windows (or Linux) is particularly bad for a control application that needs guaranteed timing and good responsiveness. The benchmarks Justin pointed you to are reporting 'average" update rates. On any desktop OS like USB and Windows, there can be lots of jitter inherent in the operating system, drivers, and services that may be running. The OS can switch to another task at any time, which messes up any guarantee of timeliness. So you can expect updates to occasionally take much longer than average.
If you need a more reliable or faster solution, I would recommend different hardware and software. A DAQmx write to a PCI or PXI device is typically 50 times as fast as the same write over the USB bus. In addition, the LabVIEW ETS real-time operating system is much more deterministic than any desktop OS.