actually, the 240w PSU inside the Optiplex 9010 is enough for a GTX 1050 Ti that does't need extra power, since it will only need 75-100W that it will get from the motherboard and the i5-3570 only needs up to 77w to perform when using turbo, along with the very small amount of power needed for RAM and hard drives 240w is enough.
So you can put a 1050Ti in it as long as it's low profile and doesn't require extra power, and then I would highly recommend upgrading the 4GB of RAM to 8GB and then you'll have yourself a decent low spec budget gaming PC.
I would advise against just putting another 4GB stick in there though and opt for either a single 8GB stick of DDR3 or two 4GB sticks that are part of a kit, not separate random sticks, if you can get them. Check your local used market to get them super cheap, they don't need to be that fast and 1333Mhz will be enough but it will support running RAM up to 1600Mhz. And anything with a higher speed will work too, it just won't run at those speeds and will default to 1333Mhz but you can go into the BIOS and enable the XMP and they will run at 1600 but only if it's higher speed than 1600, 1333 won't magically run at 1600 it will run at 1333.
So basically you can buy whatever you can find for the cheapest price and throw them in there replacing the 4GB that it currently has.