Crossfire / VRAM question

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Just starting to think about a small upgrade but will not be buying for a few months yet and just had a thought I am hoping I can get cleared up.

I am using a xfx 5850 and the plan was always just to crossfire with another when I have some spare cash. Should I be looking to match ie another 5850, would sticking a more powerfull card in cause inbalance issues?

Also VRAM, only game I have had issues with where I have thought I wish had more VRAM to play with is GTAIV. If i had two card with 1G vram each does crossfire make good use utilising both. Or would I be better off paying the extra for a card with more vram?

Thanks in advance.

 

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Cheers guys thought that would be the case about matching but wanted to be sure

 

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Ah okay, so sticking in another card with 1gb would give me a usuable 2gb in total. Sticking in a card with more vram would be a waste.