Some of you might be considering buying, or already have, bought r9 280x's. These cards have the benefits of achieving one of the best $/KH (300/700) ratios around if you can find it at MSRP ($300), is pretty power efficient (200-250 W), and it provides more KH per pcie slot than the 270 or 270x, which means decreased overhead. I have had both an MSI gaming 280x and 5 Sapphire dual-x 280x's for about a month now and I thought I'd share some of my experiences with these cards. 
 
Sapphire 280x Dual-x
I'm sure many of you have read great things about sapphire branded 280x amd cards after a cursory search on google. Unfortunately, not all of these things are true anymore for the sapphire dual-x 280x. Sapphire recently created a new revision of this card that can be identified by a newer bios (15.041). Unfortunately, the newer version is not capable of sustaining the stable undervolts and high hashrates that the previous revision is capable of. If you do a deeper search on google, you'll notice that these cards have problems with vrm overheating that causes them to throttle. This is partly due to a new vrm that isn't as good as the old one. This left me initially with a hashrate of around 700 KH/s. Luckily, LitecoinTalk user The Stilt over here has created an edited version of the bios that helps with this throttling. I used the bios that he made for this card and posted here. After the bios update, I was able to get 2 of my cards up to 715KH/s, one at 710, but the third will not go over 700 without becoming unstable. These hashrates were achieved at a voltage of 1.15V, undervolted from 1.225V. I'm still trying to find the best settings for my cards after the bios update, but keep these problems in mind when searching for a new mining card.
 
MSI 280x Gaming
The MSI 280x R9 is also highly recommended by many individuals. Unfortunately, it's voltage locked unless you flash the bios to the new version found here. Trixx and other utilities might show it as voltage unlocked and allow you to change the bar, but if you look in cgminer or a gpu monitoring utility, you will see that vdd is not changing. The new bios also takes care of the vrm overheating problems that it used to have. After flashing to the new bios, I was able to undervolt it to 0.943V without any problems. I have it running now at ~730 kH/s.
 
Being able to undervolt it from 1.2V to 0.943V means it will consume much less power than the Sapphire branded dual-x 280x. Here are the results measured on a kill-a-watt on a system running just one MSI gaming 280x video card using the original bios at 1.2V and after undervolting to 0.943V:
 
      
 
Notice a drop in power consumption of 50W. Here is the internet status screen for LTCRabbit showing the temperature, speed, and settings before and after. Notice a decrease in fan speed from 50% to 40%. I use auto-fan with a temp target of 75.
 
Before:

 
After:

 
For those of you that decide to flash your bios using atiflash, make sure that you limit the size of the backup bios downloaded by adding 20000 to the end of the atiflash save command:
 

atiflash -s [VideoCard#] [BiosFileSaveLocation] 20000 

 
 
If you don't do this, you will download a bios that is double the normal size, which cannot be properly rewritten to the MSI card if something goes wrong. If you're interested in an undervolting guide by editing the bios, cryptobadger has a great one here. Just keep in mind the size download limit command above, which he does not use in his guide. Undervolting by editing the bios is especially important if you're mining on a linux distribution such as BAMT or LTCrabbit since the AMD drivers for Linux don't support software undervolting.
 
In conclusion, I would stay away from the Sapphire Dual-x 280x until they fix the vrm throttling problem. The MSI gaming 280x is a good choice if you're willing to update the bios. I will be getting some gigabyte 280xs latter in the month and I will update the thread after I have had some time to use them. 
 
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yea the MSI cards are so marked up but they are really the best to mine on, the 270 (non x) is up to $250 + shipping right now, but I need it so I might have to pay it by the time I get the funds ready for it.
 
At least with mining it will pay itself off as you use the card, so it works out in the end.

I also I have some MSI 280x Gaming here.
I was able to reflash the cards (Original 15.042/15.041 with 15.040) with a 0.956V BUT.
The voltage is only shown as 0.956V. The card itself is still on 1.2V (Temparatures are the same, same wattage on kill-a-watt).
 
Do you have a lower temperature on your MSI?

I am also on 0.956
Which atiflash version you have used for flashing? I am using 4.17.

Could you send me your modded 0.956 and 0.943 BIOS?

which BIOS was on your card originally?
 
@Regarding Undervolting in BAMT
It works if the card is in the low performance mode ;)

Thank you I will test it tomorow. 
Software Undervolting yes
 
Which was you original BIOS on the card?

Nice setups! I have those same racks for some of my rigs. I picked my racks up at Lowes.
Reasonable priced and work well for open air rig setups.

I need some GPUs and a ventilation system first.
The room gets 4*8 rigs.

interesting racks!!!!
much like very wow!!!

I don't have the files with me now, but I will try to upload them and post a link sometime on Friday or Saturday. 

I have adjusted the link in the original post. Sorry for the delay. I had a lot going on last week and I forgot about this. The part number listed on the bios is 113-5E2490U-O46 and device ID 1002-6798.

Currently, I'm sitting at 1.15, which gives a slight decrease in power consumption and temperature, but it's completely stable. I think I was able to get it lower, but I don't remember how far down I was able to go while maintaining stability. I use a higher voltage than the lowest it can go for a better hashrate since my power is very cheap. I used VBE7 to adjust the bios voltage before I loaded it onto my card.