Because they all use the same reference design for the most part, pretty much all 5900s will be on the same level in terms of performance. What you really need to look into is the quality of the output connectors on the card, then the included software and cable bundle. If I remember correctly, AnandTech did a comparison review of GeForce4 cards a while back and in that they had VisionTek as the company using the best output components on their cards (something nVidia doesn't really look into much when designing their reference card). Since VisionTek is no longer an nVidia card-manufacturer, and BFG picked up some of their staff, inventory, and production contacts, I would assume BFG's cards would have high-quality output components. I've owned a Gainward in the past and had no quips with its quality either. And though my XFX is holding up real well, I haven't had it long enough to say if there are any problems with the card quality over long-term use. Somebody else might be better suited to answer this question.
Peace