Should I upgrade my power supply?

Are you having problems with your rig?

Coolermaster isn't the best, but if it's been working awhile you should be ok. You shouldn't need more than 550w for a single card system, maybe if you do some major overclocking. 750w is minimum for dual GPU.

Antec/seasonic/XFX are the best brands for PSU if you do want to buy a new one.

The power requirement for your graphics cards really lies on the 12V rail, if it has enough amperage to drive them well. That you would not know until you figure out what you want to upgrade to. Just about anything from Corsair, XFX, Seasonic, Anatec are good stuff (XFX is made by Seasonic). There are many more good PSU also, these are just the mainstay brands you can't go much wrong with. Get something with a Gold rating for good efficiency in the wattage your rig is going to pull.

Figure the 2 graphics cards, the CPU, add 100W for everything else then perhaps 20% for overhead on top of that. Your 6870 is around 150W, CPU around 90W, add 100W for other stuff, 70W overhead..... 410W total. Your 700W should be fine even if it's crap. But if you get a new graphics card perhaps get something better.

This is a killer Ps, a bit pricy though..
SeaSonic SS-750KM3 750W ATX12V V2.3/EPS 12V V2.91 SLI Ready 80 PLUS GOLD Certified Full Modular Active PFC Power Supply New 4th Gen CPU Certified Haswell Ready
$140
http://www.newegg.com/Product/Product.aspx?Item=N82E16817151087

Hope this helps the way to think about a PSU, remember the 12V amperage draw also, try to get something with a single 12V rail.

Just to add a bit here for you. The PCIe bus should deliver 25W per slot, but I do not count this if figuring power. If the 6870 is 150W then it draws 12 amps from the 12V rail (A=W/V), that is very little really and your PS should handle it fine. That SS-750KM3 delivers 62A (+12V) on a single rail, that should handle most any XFire or SLI set up.

If you are just getting a normal system, no OC or Crossfire/SLI, you Could do with a 500w if your GPU isn't too power hungry. Like the gtx 770 for example. http://www.tomshardware.com/answers/id-2037694/gtx-770-500w-psu.html

A cheaper 750w, m12ii by Seasonic PSU http://www.newegg.com/Product/Product.aspx?Item=N82E16817151095

Edit: if you are going the crossfire 6850 route, you might not need 750w, but that's up to you. Getting a 750-800w is a nice futureproof
http://www.tomshardware.com/forum/387690-31-what-power-supply-need
http://www.guru3d.com/articles-pages/radeon-hd-6850-6870-crossfirex-review,4.html

Shadow, everyone here will tell you what to buy as to answer your question specifically as you asked it. I like to teach someone how to answer their own question, empower you instead of just pointing you in the direction I think right, I could be stupid and telling you wrong stuff, never know around here.

I'm trying to teach you how to figure out what you need, not just tell you what I think you need and leave it at that. It's really pretty easy and it's always good for you to know why you buy this or that part, not just do what someone suggests and trust they know something. The brands jaraldo and I gave you are good ones, there are others but these are always dependable.

1. Figure your usage, Graphics Cards / CPU are the 2 big ones. Figure 100Watts for EVERYTHING else. Add on around 20% overhead to what you come up with. By a GOLD rated PS, they are efficient and can run well sustaining around 75-80% of delivered wattage. Take the CPU and GPU wattage needs together, figure the +12V amperage requirement off the rail (the CPU AUX power plug is +12V also, usually 4-pin).

Wattage = Voltage * Amperage......this simple equation is all you need.

I figured this out somewhere above and came up with 410W, you would need to go up to 450W, this would be the Minimum you could use. Jaraldo suggested a 500W, I agree because more is always better for power. If you OC your CPU you'll need a bit more power, not a lot but some, same for your GPUs, that is why 500 is better than 450. It's hard to suggest a PS for you because the one you have is fine now, when you change up then you will know what you will need. If you end up with a set of 290x's you need 300W per card, now we are talking close to 1000Watts needed. You get why it's hard for me to tell you what you need right now? Better to enlighten you how to figure out what you need when the time comes.

NE has ONLY 1 gold 700W PSU, perhaps 650 or 750 would give better choices, they have 26 @750W. But you may need more or even less, here they are anyway...15 that are SLI ready.

http://www.newegg.com/Product/ProductList.aspx?Submit=ENE&N=100007657%20600037998%20600014094%20600029781&IsNodeId=1&name=SLI%20Ready&Order=PRICE&Pagesize=20

This is the cheapest 750W I could recommend, it is made by Super Flower for them.
Rosewill CAPSTONE-750 750W Continuous @ 50°C, Intel Haswell Ready, 80 PLUS GOLD, ATX12V v2.31 & EPS12V v2.92, SLI/CrossFire Ready, Active PFC Power Supply
$99
http://www.newegg.com/Product/Product.aspx?Item=N82E16817182073

You can read a bit about their PS here: Super Flower
http://www.super-flower.com.tw/products_list.php?class=2&sn=1&page=1&lang=en

^ Couldn't agree more, well said.

Lots of my posts are infact things I've taken time to research/google. It's sometimes infuriating that other people can't google the same things and come to their own conclusions. But, I was once new to this aswell and understand how foreign it can be. The only thing I'd ask is that you at least take the time to read the links we provide as they are usually the "better" info; saving you the time of reading through 20 random threads while allowing you to learn a bit about the questions you are asking

I think you have enough info to buy a new PSU now, hehe