Sell the HD 5850 Toxic. You should get USD 125 - 150. Add another USD 225 bucks. Get the HD 7950 OC. This card will easily hit 1050 Mhz at stock volts. With sapphire trixx and voltage tweaking 1150 Mhz is achievable. Excellent price perf. It will stomp your HD 5850 CF in the latest DX1 games like BF3, Crysis 2, Max Payne 3, Dirt Showdown. The compute and tesselation performance of the HD 7000 cards is much better than HD 5000 and HD 6000 cards. In the latest games that gives it a huge advantage. Also you seem to have an idea of going for eyefinity. For such setups the HD 7950 with 3GB VRAM is a perfect fit. The HD 5850 with 1 GB will struggle in all the latest games and will run out of VRAM easily.
Here is a voltage OC HD 7950
BF 3 1920 X 1200 Ultra 4X MSAA
HD 7950 (1150 Mhz) - 55 fps
HD 6870 - 25 fps
HD 5850 is around HD 6870 perf. So even a pure 100% scaling in CF will not be enough. Also as told BF3 Ultra 4X MSAA at 1920 x 1200 will easily require 1.4 - 1.5 GB of VRAM. The HD 5850 will struggle badly with 1GB VRAM. In eyefinity the situation will get very bad for HD 5850 CF.
There are a couple of options
Gigabyte HD 7950 Windforce (900 Mhz) - USD 370
Sapphire HD 7950 950 Mhz edition - USD 390
The Sapphire comes with a ref HD 7970 PCB with 8 pin + 6 pin power. Dual BIOS. the 2nd BIOS allows aggressive overclocking. Both cards come with excellent coolers. Gigabyte has 3 yr warranty. Sapphire has 2 yr warranty. take your pick.
Here is a user who has purchased the Sapphire HD 7950 950 Mhz edition running in eyefinity
here is another user running the same sapphire card watercooled
You could get their opinion if you want.Edited by raghu78 - 6/23/12 at 3:19am