Provide link to specifications (you can find them with serial number). There seem to be gazillion HP 6300 models.
So these are:
Product number: XY107ET#UUW
Serial number: CZC1362FHV
Don't have any idea what to do with it, but as you command
That's the way it works
Then bad news. Power supply is non-standard one. Motherboard is non-standard one.
So basically you cannot change PSU and new case doesn't help as you would still need non-standard PSU.
To put this shortly, situation is much like
If you are willing to take risk, you can try card like Radeon RX 460 that "should" work with that PSU. You should still check that you can fit that new graphic card in that case. Too long cards are probably blocked.
Edited by Drillingmachine, 07 January 2017 - 03:57 PM.