If the mod doesn't work, then a 9500 pro would be better. Follow the forum link at the bottom of that page I posted to see what results people have gotten.
The reason the pro can't be changed is the reason the sapphire non-pro can be changed: rendering pipelines, and a 256-bit bus. What I mean is this:
9500 non-pro 128mb : 8 rendering pipelines, but only 4 enabled, 256 bit memory bus
9500 pro : 8 rendering pipelines, 128 bit bus
9700: 8 rendering pipelines, 256 bit bus
The card being modded uses the same PCB as a 9700, so there is a 256 bit memory bus as compared to the pro's 128 bit bus. This increases memory bandwith by a fair bit.
The cards being modded seem to have 8 rendering pipelines, but only 4 are enabled. There is speculation that on some cards, some of those pipelines are faulty (hence being labeled as a 9500 non-pro) and create checkboarding if activated. No one knows for sure though.
As for an OEM card, I don't think it really matters wether it's in a retail box or not, but the card everyone is using is called "Sapphire atlantis radeon 9500 128mb" Sapphire is the name of the company producing the card.
Basically, if you're not too sure, I'd stick with the 9500 pro, but great performance can be had rather cheaply with a modded 9500 non-pro