You want a motherboard with two PCI-E slots that support at least 8x bandwidth when there are two cards in.
Slot spacing, most motherboards will have double spacing, so this isn't an issue for most cards. If your PSU can handle two cards running at once, whether your case can hold two cards. If the cooling can handle almost double the heat output from two cards. On GPU's this old its not an issue, but if the CPU will bottleneck the cards performance.
Extra power draw, increased temps and noise. Possibly issues like Micro-stuttering, though many Crossfire/SLI users say they have never felt it.
Also its generally a better option to with a single stronger card over two weaker in SLI/Crossfire. But if you already have these laying around, might as well.
A couple of follow-up questions:
How can I know/find out if the motherboard supports 8x bandwidth with both cards in?
How big PSU do you recommend? Specifications on my cards in general says 190 watts, but due to the factory overclocking it might be more?
If it turns out to be too much trouble, I'll give it up and only use one card, but I'm thinking a motherboard capable of running both cards cant be that much more expensive, so why not try it out?
When you look in the motherboard specifications, it will say somewhere about expansion slots.
First one will always be 16x.
Second will either be 16x or 8x. Usually it will say 16x, but with a clause somewhere saying that if the first slot is occupied it reduces to 8x.
It might also say PCI-Express 16x : 2 (x16/0 or x8/x8). That just another way of saying the above essentially.
It isn't much more expensive to get a dual Crossfire/SLI capable motherboard. Any decent enthusiast geared board would have it. Basically if its above ~$110 (and non M-ATX size) you can almost expect it as standard.
Not sure on the power draw of those HD4890's. Usually for dual card rigs, 750W is the recommended wattage. but thats on modern systems with much more efficient cards. Though if this is anything to go by. http://www.tomshardware.com/reviews/radeon-hd-4890,2262...
A single card will consume ~190W. So two times that, plus a worst case scenario of an AMD CPU (125W) will get you a rough system draw of ~500W under full load through the 12v. Throw in overclocking (CPU and GPUs',gonna say an extra 60W) and mobo/RAM power (~30W, likely much less) and you have a ~600W draw under full load.
750W should be fine, and will neatly slot in at around 60-70% usage, where PSU's are typically most efficient.
EDIT: Messed up the cards draw, will recalculate.
A 750W will suffice, but you are running at ~80% usage at full load, which is a bit high. So I suggest an 850W unit.
Ahh, AMD motherboards are an exception. The chipset supports more higher bandwidths than mainstream Intel chipsets.
Yep, the two 16x slots will run at 16x when you have a card in. Should be able to fit a max of three cards in there (there are four slots, but one of them will be covered when you put in the 2nd card) with no issues of bandwidth.
Just be aware that Intel is still an option if you want to Crossfire/SLI. Z77 boards use PCI-3, whle this AMD board uses PCI-2. PCI-3 has double the bandwidth of 2, so a Gen 2 16x is equivalent to a Gen 3 8x.
Right now it doesn't matter, as Gen-2 8x is more than enough for even the most powerful modern cards, but its something to consider if you will be keeping this mobo for a couple of years.
Nice to know! I am choosing AMD because of the FX-4170 CPU. I get a CPU at 4.2 GHz 4 cores for about 165 dollars... I live in norway, so it is about 1000 NOK. Here in my country this is one of the cheapest CPU's available, and I think it will do great with my old GPU's. Intel is a good alternative as well, but a bit more pricey. I have done some calculations and think I will get a complete rig with a monitor for about 1000 dollars. This is what I can afford right now, as I am a student.
I have an old intel x-25 ssd disk, two radeon hd 4890 cards, mouse, keyboard, dvd player, so I think I save about 600 dollars using what I've allready got. Then again I have to spend an extra 100 dollars in total on a compatible motherboard and PSU, to make it all work.