our martin,
If you're looking at playing BF3 on Ultra, Crossfire 7970s is a little overkill, but yes, they'll do it just fine.
See: http://www.guru3d.com/article/radeon-hd-7970-crossfire-review/19
If money is no option, though, you're better off getting the XFX Black Edition 7970s over those Asus versions (because of their better temperature ratings while under load). You should also look at water cooling options if you plan on overclocking, but I'd suggest against it since you don't seem to be very tech savvy from the looks of your Sandy Bridge question. I'm not making fun at all; water cooling can be a pain in the ass unless you either do it yourself or buy a professionally sealed system, and for the latter option's cost you might as well just buy a second computer. Seriously, it becomes a huge waste of money unless you have an aquarium fetish or do everything yourself.
From what you'll be using the system for, don't be an idiot like all those guys who buy a ****load of RAM and never use it. You will not need more than 8GB if you're just gaming, watching movies, and surfing the web. Period. This includes any antivirus you have running in the background while doing all of the above at the same time. If you need more down the road (because of advances in technology and programs taking up more memory), then you can upgrade at that time.
Invest in an SSD for your OS drive. As big as you can get it, SATA 3, Sandforce SF-2200 and all that junk. Vertex 3 is popular (but expensive). To be honest, if you're only gaming, I suggest spending your money on a higher capacity SSD, NOT the single fastest one. Unlike someone who uses their computer for techie stuff, you will not benefit from the extra 10-20% increase in speed as much as you will extra space for a bunch of games and programs.
Also, make Sandy Bridge your only option (forget about AMD unless you are building a budget PC, at least for this generation). An i5-2500K is perfect for gaming atm, but of course the i7-2600K yields slightly better numbers for $100 more. For you, I'd probably recommend the i7, because again I don't think you should be overclocking anything unless you're prepared to fix it.
I would not recommend getting multiple nVidia cards of this generation unless you plan on investing in water cooling. They also run louder, and unless you plan on using programs which benefit from PhysX, ATI will give you more bang for your buck and be easier to implement at the same time.
Other than the GPUs, the main thing you want to focus on in your gaming build is proper cooling for your system. I build PCs myself, so I have no idea what the quality is of those guys you plan on hiring, but don't be a cheapskate when it comes to your fans and case. What's the point of buying the best **** if it overheats because you bought a low profile mini-ATX setup to take to LAN parties? It'd be the same thing as buying a Ferrari with no cup holder.