I run physx fine in my single GTX480, so you don't need a second card.
That Physx isn't used for any racing games as of yet doesn't mean it won't be. Plus who are you to decide if the good man should only stick to racing games? He might like to play Mafia, COD or Battlefield, Batman, Total War: Shogun 2, Alice Madness returns. Who knows?
I know I get bored if I have to stick to a single genre at least.
I meant you are flawed when it comes to the GTX590 in terms of price. Not the other ones. Show me a good naturally normal clocked card that is cheaper then the cheapest 580 and you win.
Also it is nice that you like to flash bios firmwares into your cards but it's not a given that this good man will want to do this either.
Plus like I said before if you have a bad profile for the game there isn't enough speed profit to justify the costs of a second card. Plus if there is no SLI or Crossfire profile there will be only 1 card used.
One thing I'll give you for AMD is that they have a better triple screen solution. Which can indeed be a better choice if you want that.
Anyhow this will be a long argument since its Nvidia vs AMD just like Forza vs GT5. I see you are someone that lives AMD cards and that's all fine by me.
I'd suggest that the good OP will have to search for himself to what he thinks will be the best solution for him and his wallet. Besides there are tons of tech sites and fora with info on this matter.
Every single modern game has SLI/Crossfire, those that don't are too old for it to matter. There isnt a card universally faster than the GTX580 in that price range, because there are no other cards in that price range currently being sold, but as I have said, for cheaper you can run 2 cards in crossfire/SLI and get better performance. With that said, in some tests/games the HD6970 outperformed the GTX580 in some situations (can't remember if it was a 1.5g or 3g GTX580). Ultimately the GTX580 is not good enough performance wise to justify its price around the other options.
Bios wiping your cards? It is an option and a popular one so why not mention it? Am I only to suggest things that you agree with? With 2 first edition (bios switch) HD6950 cards upgraded to HD6970 cards you can get the performance of a £600 (GTX590/HD6990) graphics card for £400.
To really make good use of physX you do need a dedicated card to do the work, if your single card is doing the physX then it has less power to do the graphics so you will have to take a performance hit. Also, Battlefield (BC2/3) does not use physx. Yes he may want to play other games than racing, but since this thread is specifically about a "Sim Racing" rig I don't see how that is relevant.
I don't "Live" AMD cards, Infact i've always been more of an Nvidia person. Geforce 2mx, Geforce 2Ti, Geforce 3Ti200, Geforce 6600GT, Geforce 6800GT, Geforce 7900GT. Those are previous cards, then due to reliability I moved to a Radeon HD3870 and then a HD6970, but I was very close to buying a GTX570, the extra memory of the Radeon + the performance results in F1 2010 (being higher), along with the better price made me buy the Radeon. On the other hand I could say that you're someone who "lives" Nvidia, and pushing a monopoly of your favourite card using hazy facts is not much helping someone who ultimately has to decide for themselves.
I'm pretty sure I've recommended both Nvidia and AMD cards above and each have their advantages, performance wise they are actually all very similar and you should go by price or specific game performance (that you intend to play). Toms hardware agrees with me
http://www.tomshardware.co.uk/best-graphics-card-gaming-performance,review-32301.html
Recommending AMD cards over the Nvidia all the way up to the GTX570 (first nvidia card recommended), before recommending SLI/Crossfire setups.
To the above, platform limited : Unless you haven't noticed this is a Gran Turismo forum, is it not safe to assume that someone would join a Gran Turismo forum because they are interested in the series? So I'd say that the concern of being platform limited is more than a legit one, even if he doesnt intend to play it.