Er...
If the power supply is only 250 watt, that ain't going to feed enough amps to the graphics card to accle like it should.
A new PSU may not be the total solution, but it is certainly likely to improve things. I don't install less than 400 watt in anything these days.
Changing a power supply ain't difficult - it just requires a certain patience and dexterity. I'd do that before digging further.
Now ... what vintage is the motherboard? If that's 2002 or so, mebbe the time has come!