When using multiple graphics cards, there are some important restrictions one should bear in mind, but that don't keep the real enthusiast from buying. Some of those problems are topic of subjective discussions (micro stuttering, loudness/noise, price), but have to be dealt with nevertheless. Core i7 with 3-way SLI/Quad-Crossfire: It's all about the money
Even before the installation of a SLI or Crossfire system there is a problem that already affects the users: the finances. Two Radeon HD 4870s with 1,024 MiByte already cost about 480 Euros, a single HD 4870 X2 about 410 Euros. With the Geforce combo costs can rise up to 1,100 Euros - for the graphics cards only. In addition to an X58 motherboard, an i7-965 EE, DDR3-RAM and an adequate power supply unit you can hand over several thousand Euros. Core i7 with 3-way SLI/Quad-Crossfire: Energy and Heat
You should pay some extra attention to the power supply unit. On the one hand energy costs rise because of the power consumption of over 700 watt (Quad-Crossfire) and on the other hand there aren't that many units able to provide the required power. Loudness and heat of such a high-end system shouldn't be underestimated, too. The graphics cards heat up themselves and the whole case and the background noise is also much higher than with a single card. In our opinion relaxed gaming isn't possible without a good water cooling system - even with pumped up volume because there always is a constantly "loud” noise. Core i7 with 3-way SLI/Quad-Crossfire: Micro stuttering an Input Lag
In theory 3-way SLI and Quad-Crossfire offer constantly playable framerates and only Crysis or special Anti Aliasing modes can get the fps into the one-digit area. Nevertheless the gaming experience isn't the same as it should be.
Frame distribution – before/after:This is what a solution for micro stuttering could look like. [Source: view picture gallery]
Especially with three or four graphics cards the known micro stuttering, lower the gaming experience, even at fps values of 60 or more. Even more annoying for sensitive gamers is the so called Mouse Input Lag, a delayed aiming. The crosshairs are moving squashy or jerky - clean aiming gets a little difficult. Since 3-way SLI and Quad-Crossfire are actually targeted at high-end gamers this is paradox because those users are bound to be among the first people who are annoyed by the micro stuttering and the Input Lag.
As long as you don't mind micro stuttering and Input Lags, the noise should be the only problem you could encounter with a multi GPU system.