550 ti should be perfectly fine, neither of those games are demanding in the slightest. But why don't you just get the GTX 750 ti, it costs $120 and unless you have a friend that will sell you the 550 ti for cheap it goes for ~$200 refurbished.
I don't think you can upgrade the PSU in Dell's SFF cases. At-least this is the issue with their Inspiron S series cases... But if you can't and are still wanting to build inside of it, you can always go with an R7 250 which gives off no power.