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If you DO actually have the Dimension 3000.....
I happen to have worked on a friend's Dimension 3000 in the past, and in fact I have it now - it's mboard is now no longer working properly.
The mboard has no AGP slot, so your Nvidia geforce fx5500 video card would have to be a PCI video card.
Almost always, when you install a PCI video card, the onboard video is NOT automatically disabled, so BOTH the video port(s) on the card and the onboard video port should produce video when the video card is installed (if you had an AGP slot, the onboard video would be automatically disabled when you installed a video card in that slot).
Any video card with a Nvidia geforce fx5500 video chipset ......
(recommended) Minimum System Power Requirement (W): 250W .
The Dimension 3000 comes with an el-cheapo 250 watt power supply.
If that is defective, it may not actually be capable of the running the system when you install the Nvidia geforce fx5500 card.
Your ATX power supply is always powering your ATX (form factor family) mboard in some places,including some of the contacts in the PCI slots, even when the computer is NOT running, as long as the PS is receiving live AC power.
Did you unplug the computer or otherwise switch off the AC power to the computer when you plugged in or unplugged the Nvidia geforce fx5500 card ??
If you didn't, you may have damaged the video card or the PCI slot circuits or both !
The video card can be tested by connecting it to a PCI slot in another computer's mboard.
Assuming you DID unplug the computer or otherwise switch off the AC power to the computer when you plugged in or unplugged the Nvidia geforce fx5500 card, then the most likely thing is the power supply is defective - try temporarily connecting another power supply from another working computer, if you can, that has at least 250 watt (output) capacity to your computer.
If the power supply IS defective....
,,,,,,you can replace it with any decent quality standard sized standard ATX PS with the same capacity or greater.
Standard (PS/2) power supply size - 86mm high, 150mm wide, 140mm deep, or 3 3/8" h x 5 7/8" w x 5 1/2" d , or very close to that, though the depth can be more or less for some PSs.
Don't buy an el-cheapo (in quality) PS.
See response 3 in this:
Note - I may have mentioned Coolermaster in that - I have recently found some models have only a 1 year warranty, some are known to have premature fan failures, some are known to develop failing electrolytic capacitors.