Going by the Passmark chart  LINK the GTX580 is the winner in both performance and cost.  That should light your afterburners.

if only I could afford to replace my 8800GT with one. 

PassMark are generally ballpark figures due to the minor tweaks sub-manufacturers such as Gigabyte, XFX, Sapphire, HIS, Zotac etc.. make change performance for each card. I'm running an ATI Radeon 5770 Vapor-X model (discontinued, was planning to go crossfire with this card as it worked out cheaper than a 6970 and would have had performance of the same level for less) but it still runs nicely. Do you have any links to specific video cards that have taken your interest ?

http://www.asus.com.au/Graphics_Cards/NVIDIA_Series/ENGTX5802DI1536MD5/#specifications 

That one's available where I get my 'bits'...

I remember having this debate in my head a few months ago (went from a GTX 275)... 580. The 590 costs to much for the little increase in performance. (wait what, Wizards1956's link says 590 it aint # one and its more money) DO IT!  

I have been very happy with my card. Everything maxed out and the frames are pretty nice 45-60+ for the nicer looking games.  I just went from a 24inch screen to a 40inch... Its beautiful!

Well my opinion is if you can afford the extra I would go with the 590 due to the fact that it'll handle features better in the future and would lengthen the time which you would need to upgrade again.

GTX590

Jafo i believe in you and i know you will make the right choice- but for heaven sakes don't hurt me in the process because i can't afford the gtx590 and ill be very jalous if you purchased this card.

Im rich but i aint that rich so don't buy the 590 just to piss me off? I know we have had our mis-understandings in the past but i can assure you i don't deserve to be stabbed in the back as a result of you buying this card! It aint right Jafo.. Not at all.

Over $700 per card is not something anyone will call a product. Its a collectors item. The glory of playing battlefield 3 with such a beast would put anyone else to shame who hasn't got the card. If you value our mediocre friendship then you will just settle for the 580-ok buddy?

What springs to mind for me is the monitor resolution and how much memory FSX uses. There are 3 GB versions of 580's if it would be better from a memory/rendering standpoint. If you're only running at 1920x1080 (or 1200) then you should be okay with the 1.5 GB, but higher than that and you may be better going for the 3 GB. It really depends on FSX.

this might help... i would also say the 590GTX but make sure not to get a reference model ( why ?) because they are loud - really loud !
This is just a compare about the 2 cards reference models i assume
http://www.hwcompare.com/9825/geforce-gtx-580-vs-geforce-gtx-590/
keep in mind that that there is already a big brother of the 590 out the "GTX595" im unsure but i know about two models ASUS and EVGA

Well...yrag's the boss....so it's the 590 ...

Bank balance is a little lighter .... but maybe I can palm off the 285 to a mate.  [I don't have a second machine with PCIe]...

Few things to do before I whack it in...

Well....WEI gets 7.9 and 7.9 in the graphics.....as you'd hope, I guess....

50 to 70 fps on average in FSX....this is kinda worth it...

That's 81 FPS ....but typically runs up and down between 50 and 100+ ....

As long as you're airborne, why don't you drop your old card over my place. Please use a chute rather than a missle for delivery though. The neighbors may complain. 

You can find coordinates on GoogleEarth.

If I was going to spend that kind of money, I would upgrade my CPU from it current  I7 920 D0, on a ASUS P6T Deluxe V2 to a I& 980 Extreme

I wouldn't, A newer graphics card would handle graphics/video rendering so much better. i7 920 D0 is already overclockable and I've seen some specs at 4.x Ghz. It's more than enough, despite the extreme chips having 6 physical cores It's debatable whether the "extreme" is worth the heavy price tag.