There is no yes or no answer to that right now.
Some third party Radeon boards do seem to offer tangibly lower image quality then ATi's own boards, though luckily they seem to be in the minority right now.
I'm hoping this means ATi has set rather stringent standards on output quality.
Many third party boards will have identical image quality to their ATi brethren as ATi themselves are still outsourcing a lot of boards to the third party vendors for relabling and packaging.
Should ATi decide to stop that practice then it's unknown what the usual quality will degrade to.
Some manufacturers, such as Gigabyte, and Saphire are producing all of their own R8500 boards. Hercules has recently started doing so also. Crucial's R8500 boards are going to be provided by Saphire.
The majority of third party R8500's should have decent image quality, as many are still manufctured by ATi themselves. Saphire's boards seem to have slightly worse 2D quality though.
Gigabyte's boards seem to be rather mixed, but on the whole their reasonably comprable to ATi's boards.
I can't say how the quality will be on Hercules boards, as I havent seen one of them first hand since they switched over to manufacturing all of their own ATi boards.
I've no idea who amnufacturers PowerColor's boards, and I've not seen enough of their R7500/8500 boards to state conclusively how the image quality is.
Basically, it comes done to most third party boards being safe purchases for those concerned with 2D, but not all.
On preference I'd lean against anything from Apollo or Saphire right now.
Gigabyte, and ATi seem to be the safest options. Hercules is likely a relatively safe choice also.
ATi, GigaByte,