to be more specific I have a triple and runs fine with the hardware but for- 3D -you cannot use it due to the 60 hz limitationso i have to go to 3 monitors on one card and thats the gtx 590will this dual card give lower fps?can i use as 4th screen for the extra support with the existing card gtx 460?

I can't speak to the performance difference when measured against the 460, as I have never owned one. No matter dual GPU setup you choose, you will lose performance. However, 3D requires a dual GPU, so it's kind of a moot point. A pair of GTX 580's will give superior performance, due to the difference in the clock speeds on the two cards(however a single GTX580 would give better FPS). The 580's come clocked at around 800MHz, whereas the 590 is clocked at only slightly over 600MHz. I have seen several users on Overclock.net who have overclocked their 590's to well over 800MHz when they use water cooling, so it is possible. Nvidia says it's unsafe to make more than small increases in the card's voltage without liquid cooling, so if you want to match dual GTX580 performance, liquid is required. As far as I know, it's totally possible to use a second GPU for things like moving maps and other applications. It isn't ideal for performance though, as you will be diverting some of your precious CPU horsepower to running those applications. To run things like RC4, ASE and EFB a low powered second computer will improve the performance of both your external applications AND FSX.

ok but 2x gtx 580 is not an option because i want 3 screens plus 1 for application and also 3das far as i understand now, i have to wait till gtx 590 will be replaced by one with more clockspeed or i need as you said a watercooler.so i might work although i got the feeling that i am in a risky field regarding the FPS and processor and the housing(its overloaded and gives heat problems if not removed)so may be i will wait another year.last question is the 590 2000 or 1000 mb per card.

If your objective is to have a card that's a duplicate of two GTX580's, yes you'll have to wait for a faster card to be released. FPS are gonna be risky on any dual card setup, regardless of the clock speed.The GTX590 has 3GB of VRAM, 1.5GB per processor.