It will not hamper it. Readyboost is parallel. It will request information from both your hard disk and USB drive at the same time and will use that information from whichever gets it 1st.
If you're interested in seeing if it's of any use, you could do a blind test on yourself. I once had an Intel SSD in a USB enclosure and used it as a Readyboost on my work PC with 4GB of RAM. I was suprized at the difference.