GX2 is a great card I own one.
As for under performing I wouldn't say that it's not as fast as some of the later cards but it's a VERY respectable card.
I've had mine now for 2-3 years I think now great card only prob I had with it is it died on me yesterday I stripped it down and baked the PCB's and it's good as new again after new cooling paste and reassembly.
I'm a VERY heavy user and a avid 3d gamer and I can't say I've ever had any concerns with it's performance even in the heaviest 3d games.
Its a dual GPU card which has it's advantages depending on what you want to do with it.
Personally I have one with a 2kw PSU it never hurts to invest in a good quality PSU with stable voltages.
It may not be a 580 but then considering it's age its a brilliant card.