From your post, I gather you have a visa from Feb 2005 that is valid for 6 month from USA to china? Now, you are coming to HK in May 2005, and want to go into China FROM HK. Is this correct?
If so, information for you is:
1) Depending on the type of Visa you got in Feb is WHAT kind. If it's a tourist one. Then you have no problems. If you got a special Visa ... for example, like a work visa, or the F visa (cultural exchange), and your reason of going into China is THE SAME .... then you should be ok as well.
However, if you got a work visa last time, but no longer employed or supported by that company, and this time you come in May as a tourist, then it won't work very well, you could try, but a little risky.
2) The Visa to China does not indicate WHERE you depart, it's a Visa into China, doesn't matter whether from USA or HK or whereever.
3) Make sure you have a visa that allows you to have multiple entry into China. B/c the visa that is single entry cannot be used again.
Hope this helps you a little.