An Nvidia 710 GFX card will work well with a 240W PSU. An Nvidia 730 GFX card (much better, get the ddr5 version) might work OK. Beyond that, I don't know. With current GFX card prices, I don't know if that meets your budget.
Your limitation is the PSU, and possibly physical constraints. Since you did not provide additional info, that's as far as I can go.
FWIW, the 710 on a Win 7 PRO computer, with max RAM, with a vintage Intel Core 2 Duo processor will render even modern flight sims at very respectable frame rates, while requiring few user-selected graphics downgrades.