It's not good to push the PSU to its limit, it will become unstable.
If you buy a totally new GPU, and use it by itself alone, your PSU will be perfectly good to use. If you buy another 290 to use in CrossFire, you will be pushing it close, and I would recommend a new PSU.
The 4570 will be great for 2-way Crossfire, but I don't know what you have now to compare the performance difference.