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Go Big or Go Home
Multi-GPU video solutions have been around for awhile, and for most of that history, they've been completely pointless. Using two video cards (or two GPUs on one card, like the recent Radeon 4870 X2) has always been a less elegant proposition that it might at first seem, particularly if you're used to Dual-Core and Quad-Core CPUs, which seem to just plain work, no fussing with drivers or settings. Using multiple GPUs has never seemed to be as simple, and has never seemed as effective, then using a CPU with multiple cores.
Unfortunately, this is still partly true. But Multi-GPU solutions have come a long way since the dark ages of linking up a pair of Voodoo2 cards. Over time, the drivers responsible for making multiple GPUs play nicely with one another have become better, and they've begun to work in a wider variety of games. Today, virtually every high-end title can take advantage of multi-GPU gaming rigs. But there are still drawbacks to consider, and since a multi-GPU setup can be quite costly, you need to know the details before deciding if multiple GPUs will work for you.
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Can You Handle the Power?
If you decide you perfer the green team, then you will need to buy a motherboard that is compatible with SLI. This means you're going to need to buy a motherboard with an Nvidia chipset (unless you're planning to build a Skulltrail rig). This is a bit of a bummer, because Nvidia's chipsets arn't particularly impressive, and also because the average price for motherboards with SLI is a bit high. Going with ATI nets you a few advantages here, because CrossFireX is available on Intel's P45 chipset. The P45 chipset is a real workhorse, and like anything that gains a reputation as a workhorse, it isn't very expensive.
You will also, of course, need to buy two or more video cards. If you're going with Nvidia, then they will need to be the same card. If you are going with ATI, you can actually use two different sorts of cards - for example, a Radeon 4830 and a Radeon 4850 - but it is advisable to simply buy two of the same card for the best result.
In terms of items to purchase, this may be the only new part you need. However, it is also important to make sure that your current system will be able to handle multiple video cards. These cards are large, and if you have a small case, the cards may run into your hard drives. A small case may also cause heat issues, as having two cards will obviously produce more heat, and the cooling system in a small case may be overwhelmed. And last, but not least, you'll need to check your power supply. A high-quality PSU of 500 watts from a reputable brand is a good minimum for two cards, and 600 watts is a minimum if you're planning to run two cards that are top-of-the-line. If you're planning to run more than two cards, then it is time to start looking at a 750 Watt power supply. Also, make sure you have appropriate power connectors for the cards. Keep in mind, when thinking about what your computer can handle, that these upgrades are expensive and may make Crossfire and SLI much less worthwhile.
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It is generally true that when you spend more, you expect to get more. But it is also often true that the more you spend, the less value you get out of each dollar. Multi-GPU solutions are no exception.
When considering a multiple card setup, it is important to remember that video cards, even in the best of situations, do not scale linearly. If you can run Fallout 3 at 30 frames-per-second on your current video card, adding a second video card isn't going to increase the framerate to 60, and adding two more won't increase it to 90. It is far more likely that adding a second card will get you a gain of around fifty to seventy-five percent. This makes sense, as some degree of reduced performance per card is probably unavoidable, as your graphics cards have to process how to split up the work, and they aren't the sole component which determines how quickly a game runs.
If you're expecting double the performance, then you will be disappointed. Using a multi-GPU solution means getting less value out of your hard-earned money, at least if you quantify value as framerate-per-dollar, so value shoppers will never find multi-GPU solutions worth the extra cash. However, if you are going with a multi-GPU configuration not necessarily to see a linear increase in performance, but because you want the highest performance possible, then you will be satisfied. It is impossible to get higher performance with a single card than with multiple cards, and for power-hungry enthusiasts, a multi-card setup is worth every cent.
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Summary: Performance Vs. Value
In a nutshell, the argument over the worth of a multi-GPU setup depends on your priorities and on the capabilities of your current hardware.
If you are looking for maximum performance, no matter the cost, then buying multiple cards and running them in SLI or CrossFire is an excellent option. The performance that these configurations offer is unmatched - two Radeon 4850s, for example, are generally faster than a single GTX 280 and much quicker than a single Radeon 4870.
However, if you perfer to get as much power out of each dollar spent as possible, then you are better off buying a single, high end card.
You will also be better served with a single card if your computer has several components which would restrict your ability to use multiple GPUs. This is true even when performance is your primary goal. If you find yourself with a computer which is too small, too hot, and lacks the power to run multiple GPUs, you are probably better off thinking about the purchase of multiple GPUs as part of a complete system rebuild, rather than an upgrade.