Need advice on crossfire motherboard

[taruncharles]

Hi,

I am building a new PC for my gaming, internet surfing and other basic home usage. And I have choosen the below parts
CPU - AMD Phenom II X3 720BE
GPU - Sapphire ATI RADEON HD 4850 512MB GDDR3 (Interface: PCI Express 2.0 x16) - http://www.canadacomputers.com...018942&cid=999.243.272
Motherboard - GIGABYTE GA-MA790GP-UD4H AM2+/AM2 790GX

I am planning on using a Crossfire setup with another ATI RADEON HD 4850 512MB.
The above Gigabyte motherboards specs says this about the PCI Express 2.0 x16 Expansion Slots
1 x PCI Express x16 slot, running at x16 (PCIEX16_1)
1 x PCI Express x16 slot, running at x8 (PCIEX8_1)
I am aware I should fit-in the first 4850 512MB in the first x16 slot, running at x16 . Now, can I fit-in the second card in the second x16 slot, running at x8. Will this work?..Will there be any performance issues here?..Or should I use another board with 2 x16x16 slots?..Or can I go for another board with 2 x8x8 slots?..
Can you suggest a DDR2 board where I can run the above Crossfire setup successfully?..

Thanks a lot guys..

 

[lopri]

The two PCIe x16 slots on your board shares bandwidth. When the 2nd PCIe x16 slot is occupied by any PCIe device, the two will operate at x8 bandwidth.

Originally posted by: taruncharles
I am aware I should fit-in the first 4850 512MB in the first x16 slot, running at x16 . Now, can I fit-in the second card in the second x16 slot, running at x8. Will this work?..Will there be any performance issues here?

Yes, you can do it and like you're aware the card will operate at x8. With a HD 4850, performance loss (if at all) will be negligible. Some synthetic benches like 3DMark06 might show 2~3% less points, but that's about it. No reason to go for more expensive boards.

Welcome to AnandTech.

 

[heyheybooboo]

Gigabyte GA-MA790X-UD4P 790X - same mobo (essentially) without the IGP

In Crossfire mode each PCIe x16 slot will operate with *x8* PCIe Gen2 lanes.

You appear to be confusing the terminology a 'bit'. The physical size of each slot needed for your video card is "x16". In single card mode the x16 slot will operate with 16 PCIe Gen2 lanes at a data rate of 500 MB/s per lane for a total theoretical bandwidth of 8 GB/s.

With the addition of another video card in the second 'physical' x16 slot the total theoretical bandwidth of 8 GB/s is split between the two x16 slots - each slot gets 8 lanes, or 4 GB/s.

Depending upon your gaming resolution 4GB/s is typically more than enough bandwidth.

At higher resolutions (let's say 19x12 and up) it starts to get a little more tricky. In heavily textured games with the extra screen real estate the additional bandwidth (and 1Gb card memory) of each slot running with 16 lanes can see a 5-10% improvement.

For 2 physical x16 slots with 16 lanes each you will need to look at the 790FX chipset motherboards.

 

[taruncharles]

Really thanx a lot..Ur replies cleared what I was confused about..
Just one more clarification...Two x8 lanes wont degrade my Crossfire setup..
But, on the other hand, now I am thinking, will it really be more effective than one card in x16?..
Do you by any chance know how much more percentage I would be able to gain with a 4850 Crossfire setup in this mobo?..
Should I go for the crossfire setup, is it worth the extra card's price or should I just stay with one 4850 with GA-MA790GP-UD4H mobo?..
I am going to use a 19" monitor..So I may not use resolutions more than 1280 x 1024 or 1440 x 900..

Thank u...