your really changing up where the 12 volts are coming from when you use molex "cheaters" a.k.a. "adapters"
without getting involved in a explanation of the anatomy of a PSU; your putting a large load on a 12 volt supply that was designed to support a small load going to a hard drive or other peripheral.
you may get away with it with one gfx card that isn't going to cause a large load. but something more substantial like a gtx 470 is a whole different ballgame.
especially when you are using a "less than high quality" PSU.