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.
To handle a multi-GPU solution, you first need to decide if you want to go with Nvidia, and use SLI, or with ATI, and use CrossFire.
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.