Looking to upgrade GPU

without replacing the power supply unit: gtx750ti

I don't know if your power supply has the pcie power connectors for a more powerful graphics card or not nor which processor you have but you theoretically could run a gtx980 in that quite easily.

Whats your total budget?
 
You have to see if your power supply has any 6- or 8-pin PCI-e power connectors. If so, you could run any card requiring as many PCI-e connectors as you have, up to the power limit to the power supply.

Since it's under 500 watts, you'll want to either get the most efficient card you can get or else buy a new power supply along with your graphics card. I'd recommend buying both at once, though. Everything else in your system is fine.
 
The HP site you linked indicates you have a 460W power supply. Could you provide the actual ratings on the power supply and if it has a PCI-E connector at all? You can easily run a GTX750Ti model that does not have a PCI-E connector. If you want to run anything more, you probably need a PCI-E power connector, which you might have.

If you upgrade the PSU you can probably install anything you want to.