If you want to upgrade, the PSU should still be enough (there`s no way to predict the power consumption of future GPUs, but the trend is to lower it). Now, if you want to ADD another card, you shoulf get a 750-850W PSU.
Also, the AMD cards usually require more power than the Nvidia ones, so it`s up to you what you decide to do in the future.
The SSD is not something you should worry about when thinking about the PSU. The main consumers are the CPU and GPU.