I have Win 7 rc x64 on my new build and am generally VERY happy with it.
However, it is NOT as stable as a production release and, from what I have read elsewhere, seems to be a little less stable than the Beta release.
In practice, this may make sense. For the beta, Microsoft released everything that they had finished and were primarily concerned with checking for useability issues that would require changes to the functionality. Then, for the RC, they made changes to the functionality and added some other features that had not been ready for the beta. For the RC, they are only really concerned with fixing bugs and so the RTM will probably have the same functionality and features, but will have the main bugs fixed. Thus, they went from a stable 95% solution to a less stable 100% solution, I suspect.
In your case, assuming that this is genuinely for a gaming machine and does not have anything which would cause problems if it got lost, I would recommend using Windows 7 RC. It is generally fine and avoids the risk of paying $100 for Vista only to then need to pay another $100 for Windows 7 when it is finally released. Of course, if Microsoft would extract its head from its tuchus and advertize that upgrades from Vista bought from now on would only cost $10, the decision might be different.