Should I replace my 5850 Black Edition with a GTX 480?

It depends on what you want to play, if you want physX, etc
Have you thought about Xfiring 2x 5850s? Your PSU can handle a GTX480 or 2x 5850s.

Are you trying to max out Crysis at 1080p and Ultra High settings?

Btw, if you plan to sell your 5850s, I am willing to buy them.  

Your board should run 2 x 5850 at 8x/8x which is a small hit but still faster then a single 480GTX and would use alittle less power then the Nvidia card.. the best thing about CrossFire is in 2D you only use one card so the power draw is min and once you fire up a 3D app then the second card wakes up so your not pulling the total power of 2x5850 but only when needed.

Also you can find used 5850's around $225-250 which would save you enough money to try Eyefinity if you can find some cheap LCD's.. I run BC 2 at 5760 x 1080 in high settings with my CX 5850's on an Evga x58 e758-A1 board and the i7 930 cpu and it's a force in surround gaming..

I agree upgrade your Mobo....Asus makes really good one's for the 860......and in a simple answer NO the 5850 Black is a Sweet Card !! The 480 run to hot and Use up too much power

Yeah... I love the GTX 480, but it would just be silly to upgrade from a 5850.

You can light a fire inside that 5850 and overclock the s*** out of it if you haven't already, it's pretty impressive.

But, if you're planning on keeping your CPU for another round of graphics cards after that... buying a new motherboard would be worth it to me... then Crossfire.

EDIT: Or, I guess if you could sell your 5850 for a real solid price, who the hell cares, get a 480 if you want.

I would suggest you overclock that HD5850 as much as you can, If you can get it to 1000+ Core and 1200+ Mem It'll be slightly faster than a stock HD5870 and very close to a GTX480, However you would need some good cooling so either something like this: http://www.overclockers.co.uk/showproduct.php?prodid=HS...

Or a full blown water loop.

Thats my suggestion, I guess if you want the advantages of an nVidia card like PhysX and Cuda and you can get a good price for your HD5850 then by all means go for it.

I guess you're not really on a tight budget eh?