Question about Crossfire with APU

I mean...using a discreet card with an APU. About, maybe 2 years ago, I built a system for a friend, similar to the 'Kill Your Console' rig. I managed to save some money by buying an A10 5700 off of Ebay (it worked out half the price of the 5800K as I won the auction at 5am).

 

Anyhow it has the 7660D the same as the 5800k but clocked a bit less. 384 streams. He has been using a 6570 1GB Asus card off ebay (cost £14 in an auction) it does benefit most games, but still is too weak for 1080p.

I can get an R7 240 with GDDR5 (72gb/s) bandwidth vs 28 of the 6570, for £30 secondhand.

 

When using crossfire, does the APU use the memory of the card ? Or does the system default to the ram ?

I figure the ddr5 bandwidth if it could be used by the APU in crossfire, would boost games significantly, even though the card is quite weak itself, with 320 streams.

 

 

The RAM is shared. Data on both the card is copied on to the the system RAM. So you will still always be limited by your system memory bandwidth. 

I assume running the two GPUs together would increase performance a bit but not insanely so. Dual graphics really isn't a very good solution. Save up and just by an R9 270. 

Also I'm not entirely sure if you can even run dual graphics with the 240 on that APU... I thought that was only with the 7850K. But I'm not sure. 

Only the Kaveri APUs can run hybrid crossfire with the R7 240, the 7850K can only crossfire with the 250. The 5800K can only do crossfire with the Radeon HD: 6670, 7570, 7670

I thought the 7850K could work with the 740 too?