I have Dell Optiplex 780 PC With Power supply of 365 w, 16GM RAM & the Processor is intel(r) core(tm) i5-2500s cpu @ 2.70ghz.
Please suggset a good ghraphir card for this system for playing heavy games.
Intel does not make graphic cards. You need to explore cards that use AMD or NVIDIA graphics, or visit a site like TomsHardware to compare graphic cards.
Also, your processor is rather old. And, it is not supported on Windows 10, nor is it suited for heavy game usage. You should also look at getting a system that is much more current than what you have. Lastly, if you were to get a upper end graphics card, it would likely not be compatible with your existing system, nor would your existing power supply support it.
Bottom line - Get a new system.