two graphics cards:
GIGABYTE Radeon R9 270 2048Mb
if possible, then how?
crossfire will run to the lowest card so if the cards are very far apart the slower card will slow down the faster one and may make things slower over all.
crossfire works best with identical cards.
but if the cards are close enough even without crossfire some games and most boinc volunteer computing programs
can and will use all of the cards that are plugged in. I once ran 2 5970 and 2 6870 cards to run seti@home in the same computer so it depends on what you want to do.
if I want to add up the memory of graphics cards, ie 2048 + 1024 , can i get this?
no but the straming cells grouped by card can do lets say 2 cards 2 boinc tasks or 2 different things in a game
No, you cannot. As a general rule, if the first two digits of the card model are the same, they can be CrossFired together. With new generation rebadges, you can do a bit more.
The R9 270/X cards, for example, are essentially renamed 7850/7870 cards, and can be CrossFired with them.
That 5570 is two complete generations behind.
No game I've heard of will use multiple GPU's outside of driver-controlled CrossFire and SLI. DirectX 12 can theoretically change that (and allow using multiple GPU's across brands, even), but I'll believe that functionality when I see it.
GPU compute applications are another thing entirely, and have nothing to do with CrossFire.
Unless you plan on using a large number of monitors (more than the R9 270 alone can drive), you stand to gain nothing by trying to use that 5570 card.
basically even without crossfire if the difference is more that 10 to 1 even things like BOINC will not register the slower cards
as to the first 2 digits you would have to look much closer at the cards specs to say whether or not that is possible.
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