|There are three possible case versions of the Optiplex GX520 series - Small form factor, Desktop, or Minitower. The first two of those have 220 watt power supplies; the Minitower has a 230 watt power supply. |
The Radeon 9000 video chipset requires your system has at least a good quality 250 watt power supply (as in, it's actually able to supply enough current - amperage - at +12v to support the requirements of the system and the video chipset on the card when the card is installed).
(I have a Radeon 9200 video card - it will work on a system that has a good quality 250 watt power supply, but not on a system that has an el-cheapo 250 watt power supply, or a good quality 200 watt power supply. Dell systems, and brand name systems in general, tend to have el-cheapo power supplies. El-cheapo power supplies tend to have lower maximum amperage ratings at +12v, or the ratings are a lie and they can't actually supply that much amperage.)
Detailed info about Second Life-compatible systems
ATi (9000/8500 series Radeon cards) and Nvidia (FX/GeForce4/GeForce3/GeForce2) run on any machine with 250-300 watt power supplies.
Diablotek Radeon 9000 Video Card - 256MB DDR, PCI, (Single Link) DVI, VGA, TV Out
Minimum System Requirements
350 watt or greater power supply recommended.
If you want to know if replacement power supplies are available that have a higher wattage rating
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At least the Small form factor model (skinny - too narrow for an optical drive to fit across the width of the case) has an oddball proprietary power supply
The Desktop power supply is probably also oddball proprietary but has a different shaped case.
The Minitower may have standard sized power supply.
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.