Running two cards at the same time to increase performance (called crossfire for AMD cards and SLI for nvidia cards) is commonly used by gamers who want to have absolutely the best resolution (highest across multiple monitors); best AA (anti-aliasing to smooth lines) and FPS (frames per second to give smooth runs). The disadvantages typically boil down to problems with the drivers (especially with AMD crossfire). Frame stuttering, setup difficulty and the general hassle may or may not be worth the time required.
IMO, I would always buy the BEST single card solution I could afford. Both AMD and NVIDIA make great cards, and the higher end cards are capable of playing any game at virtually any resolution with FPS of 60 or more which will yield a movie like quality. SLI for the TI boost can be done, but the gains and issues you may encounter are more easily solved by going with a better single card like the new gtx 770.
All that said, the 650 ti boost is a really good card, and in sli compares very well with the 770 or radeon 7970, both of which are more expensive than 2 650's. Nvidia has better multi-card drivers at this point, so the sli of 650 ti boosts is not a bad option or price point.
Tough choice - but in this case I might make an exception to my single card rule.
Marik
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