As the title states I am trying to decide whether it is worth it for me to get a second 6870 and crossfire. I currently am gaming on a Dell 3007WFPI Monitor and would like to get my games (D3 BF3 TOR EVE Skyrim et al) to run smooth at higher graphics settings at the 2650x1600 resolution.
My current Computer:
ASUS Sabertooth X58 LGA 1366 Intel X58
Intel Core i7-950 Bloomfield 3.06GHz LGA 1366 130W
CORSAIR XMS3 4GB 240-Pin DDR3 SDRAM DDR3 1600 X 3
SAPPHIRE FleX 100314FLEX Radeon HD 6870 1GB 256-bit GDDR5
Thermaltake Toughpower W0132RU 1000W ATX12V / EPS12V
At their current price point and their scalability, I would say go for it. They're nice cards, too, and I'm sure that 2 6870's blow anything else away unless you're spending $400 on a single card like you mentioned.
You wont know your temps until you get them set up. The amount of airflow your case has may determine how effectively your cards will be cooled.
You may want to consider a reference design for one of your cards, so that it blows hot air out the back of your case if you have poor airflow.
I'm currently running 6870 Xfire. I can blow anything away in 1080p, but once I turn EyeFinity on, boy does she dog. I can get playable FPS for BF3 on medium settings, but that's about it. VRAM has to be the culprit. I've been thinking about selling them now and getting a 670 or 7950.
And, my reference design card runs 15C hotter than my non reference card.
In my opinion, it will be a significant boost to what you see now, but you may not be able to stretch their legs on the resolution you want. If you can't sell what you got and swing for the $400 ones, grabbing the second one is the next best thing.