On my PC parts checklist I note some important components for building a computer including a power supply and cooling, which need serious consideration for building a computer you're going to use for both gaming and folding. If you're building a gaming system, you'll want to use the "idle" setting for Folding@Home so it won't slow down your computer gaming experience. If you're using your computer for gaming, you're going to end up needing to buy a higher wattage power supply and think about cooling carefully, so you don't overheat (or burn out) your components.
To clarify the terminology, a folding rig is a computer you've build (or modified) for the primary purpose of contributing to the Folding@Home project. A lot of people recycle old computer parts for a folding rig, just upgrading a few things like their graphics card. Folding rigs using a graphics chipset for folding typically don't need a huge amount of RAM or a super fast CPU. While Folding@Home comes ready to use your CPU for folding, most people find their electric dollars better spent focusing on an efficient but speedy graphics card.
When building a Gaming Rig, that will be Folding in its idle time, you're going to want to build a system where Folding specs is considered as a second to gaming needs. I really need to note here, that if you're going to use a gaming system for folding in idle time, and the folding is at Full Speed, it's going to cost you a tremendous amount more in your electric bill than building a Folding optimized system. And you can always build a Folding Rig to use as a gaming sytem, but the most serious gamers might not like that experience as much.
You can install several graphics cards with one motherboard as long as your CPU, RAM, power supply, and cooling can handle the system. I should note that serious Folders would suggest you configure a system with multiple cards differently than you'd configure it for gaming. Using a multi-card system would generally be better than using two separate folding machines if you're building a system from the ground up. If you're using multiple computers, you should configure your username, passkey, and team number to ensure you get all the point credits you deserve.
As I noted, if your using a computer for gaming, you'll need a great power supply and cooling, and your electric bill is going to be higher if your computer is running all day. (Might want to consider a UPS, backup power supply, so it doesn't mess up your fold if your electric power cycles.) If you're building a folding rig, you can explore using multiple cards and consider the watts used to get the most points per watt. You're likely going to pay at least USD $600 to build an efficient folding rig, but a high-point system can easily cost $1200 or more.