The Nvidia driver is free only in the sense of 'gratis', but its use is restricted in other ways. Because Nvidia have been reluctant to release specifications, the nouveau driver has had to be reverse engineered, and currently lacks 3D features that are in the proprietary driver. It may be some time before it reaches the same capability as the proprietary one, but it has the major advantage of having a free licence.
If your aim is to play 3D games, then in many cases you are stuck with having to install the proprietary driver. I have found that Nvidia has started to drop support for the really old cards in Linux, so there are some cases where nouveau is the only choice; but for newer cards you may still prefer to use the proprietary solution.
It may also be that your problems with starting X are not related to the nouveau driver; if you have more detailed information, you could post a new question. If it really is nouveau causing the problem, then a careful bug report would be appreciated so that the nouveau code can be improved.