Can i run ppsspp with old GPU ???

[LunaMoo]

Yes, but only using d3d9 backend which has worse compatibility, doesn't support post process shaders and overall support for that backend might be dropped completely sometime in the future.

If your hardware is very weak, you will likely be forced to use low resolution and not be able to achieve full speed in more demanding games or more demanding places of most other games. You'll have to try for yourself to be sure, commonly more popular titles are the most demanding ones, but some less popular titles can run on pretty much any potato.