It's the last decision I have to make before I start ordering parts, and it's bothering me.
Before yesterday I was set to get the $400 5870 2gb 6DP card from NewEgg, which seems like an unnaturally good deal. Usually it's a $550 card for the 2gb version. I live in Canada so I'd be shipping it through my fiancee in the US. I wait an extra week or so but it saves a lot of money.
Then yesterday I'd been encouraged to look at dual 6870s instead, as their architecture is newer and they're quicker cards. I'd had dual 4850s in my last rig so I'm no stranger to Crossfire. The 6870s only come in 1gb so 2 would even it out and I'd end up with something faster.
There are wrinkles though, that drag me around. Modern graphics cards all take up two ports. When you consider the requirements that this imposes on a motherboard, I would be unable to fit a sound card and network card in most motherboards; the cards would be covering everything up. I might be able to physically fit two cards on the board I'm looking at, but I'd have to get a PCIE sound card, and the pickings aren't good in that category without getting expensive (I'm not an audiophile).
Besides the physical implications, the two 6870s together would cost ~$140 more net, and there's always places to invest that money. The 5870 is older, but it's only one card and it's cheaper. The two 6870s would be faster and newer,but more expensive and more taxing on the system (I'm aware that each individual card's power has been tuned to be more energy efficient).
Crossfiring modern cards seems entirely impractical right now (honestly how do other people get around this? Why do mobo manufacturers even put slots next to PCIE2 slots?) and saving money is always a good thing. I'd be using 2-3 monitors (Two of my monitors are 24" and 16:10, the third being 23" and 16:9, so something would be screwed up if I tried that no matter what) so the power would be put to use. I have a 750W PSU and my case is large enough to accomodate either choice. I'm leaning towards the 5870 2gb right now. I'd rather now wait a month or more to see the 6900 series debut and work its way through its first wave release, then get inflated for Canadian pricing, and end up being $200 more than the 5870 2gb.