1. No.
2. You would need to add the same amount of maximum wattage it takes to power the first card.
3. Memory and fans? Practically none unless you truly have a fuckton of them. Allocating 50w would be reasonable. CPU? Really depends on whether you're overclocking or not, how many cores you're pushing, etc. Some desktop cpus take 65w or less, my overclocked, overvolted opteron over there is pulling down approximately 150w at full tilt.
4. You could try it. If you don't have enough amps to push what you're doing, it won't be happy times.
I think you're asking a lot of the wrong questions, so I'm going to bother to tell you the things you didn't ask for. When you're building a PC, the first thing you need to do is figure out your general needs by using a psu calculator like
this one. That will tell you what your theoretical system is going to pull down. Turn up all the TDPs to max and don't forget to overclock if you swing that way. I just specced out my PC on there and got 335w, which is pretty damn accurate since I also just ran Dragon Age (which maxxes out both cores and the gpu) and my UPS told me I was pulling down about 325w. At an idle, my machine pulls down about 238w, which is an important statistic for fan speed noise in relation to power supply draw, but that's sort of for later.
Whatever that number comes out to be, you want to add at least a couple hundred watts to. Why would you do that? So your psu isn't running at 100% all the time. That's hard on the psu and makes them irritatingly loud. SPCR's research in power supply loudness per 50w of load tends to suggest that the volume increases exponentially somewhere north of 50% load, so for a system like mine, a 550 to 600w psu is a reasonable choice for quietness. Also, that allows me plenty of headroom to upgrade. Technically I could scrape by with a strong 450w psu, but I wouldn't want to add anything to the system.
Let's say I had a motherboard that supported SLI, and I wanted to slam another 9600GT in here right now. How many watts does my video card pull down?
GPUReview probably knows. 95 watts it says, so if I were to theoretically run SLI on this machine, my system would pull down 325+95 = 420w under gaming 3d. Now that 450w psu would be really straining, but the 550 - 600w PSU would be right in the sweet spot for maximum efficiency, perhaps at the cost of some decibels.
Now, there's more to this story than just watts, and back in the day we all used to kick faces in at least once per day talking about amps amps amps, because you need a specific amount of amps (maybe I should say amperage to piss off all the EEs like noob master) to push your video card. The suppliers only recommend power supplies in terms of wattage because people are stupid and typically if you buy a power supply of high enough wattage, it will have enough amps to push your card. But it might not have enough to push two. And it might not be able to push one big card later. So what you need to bother to do, is figure out the amp draw on the +12 rail of your video card. How is this done? It's not as easy as the watts because some companies don't publish it. Search for your card from multiple manufacturers, and one of them is likely to have it listed. Newegg is useful for this, and occasionally lists the amp requirements in the specifications of the product. I couldn't find any info from XFX for the amps, but gigabyte off newegg says they want 24amps, evga's website doesn't have a 9600gt listed anymore, but they have a 9600gso that requires 18amps and a 9800gt that requires 24amps. It's pretty safe at this point to assume that my card requires at least 24amps on the +12 rail.
Reviews from enthusiast sites also tend to list amp requirements, and it's easier to catch those on recent cards than some old relic like mine. For SLI, apparently I used to say double the amp figure from one card and roll with that. So to run SLI 9600GTs in my system, I'd need at least a 550w psu with 48a (combined) on the +12 rail(s), which I do not have. The NUUO puts out a maximum of 38a, and I'd be most likely fucked. What's worse is, the NUUO was even SLI certified. However, it was certified for two 6800GTs, which pull a maximum of 19-20w, riding right up on the maximum output of my old psu (20+18 = 38a).
So there. I think that fills the gap between what you were trying to say and where you wanted to go.
