Absolutely right Gudster, I would do an update if a) I had the time, and b) If i still kept up with vid card technology etc. But as it is so progressive, it's difficult. All you need to remember is that Nvidia and ATI, usually have a 3 tier card range, aiming at the budget, value concious market, the mid level general market and the advanced/high end market.
Just remember a 5200 is an entry level card, a 5600 is the mid, and 5900 is the top end. Basically newer released cards still follow this formula, so just check to see what the next evolution of card is in each category. ok this may not be of much help and if anyone wants to write a performance/compatability guide update, feel free. I am sure the mods will make it a sticky.