Im running an old dell optiplex, its my main windows gaming rig since my most powerful machine runs linux. Plopped in a gt 1030 and bam, runs fine, fits perfectly. The 1030 should be able to run roblox like a breeze, and because of the low wattage chip, sip power from the PSU with no concern for using up too much power, since the chip runs at 30 watts, and not a full 75 watts which according to most forums, OEM pc's struggle to provide through the pcie slot. Gt 1030 should work in your case, and I personally recommend this one from ZOTAC, as i've used it
Be careful though, DO NOT BUY a GDDR4 version, as those run like crap. If you do choose to buy a 1030, please ensure that you are purchasing a gddr5 vesion, so that you do not waste money because of nVidia's utterly horrible penny pinching scheme. (the one linked above is gddr5)