Alright. So first, yes that would be a good video card. If you're looking for the best performance, however, instead of a 7800GT you would want a 7800GT
X, which would cost a little bit more, but right now it's the best gaming graphics card on the market. eVGA cards get very good reviews generally, so
this is one I would recommend.
My point about the motherboard you picked was that, for your uses, it had
way too many features. The most important one is that it is SLI capable. SLI stands for Scanline Interleave Mode. What it means, however, is that you use
two graphics cards in conjunction with each other to render the image faster. However, this is really only for people who are looking for the absolute top of the line, and cost is no issue. If this is you, great, get two 7800 GTXs and spend $5000 on your machine. If not, you don't need it. Save a few dollars by getting a motherboard without the SLI capability, and instead put that money towards a better graphics card. You'll get a much better bang for your buck that way.
Instead of getting that particular model, get a motherboard with fewer slots and fewer extra features and save some money there. Get one with a single PCI-Express 16x slot, rather than two. Something like
this. It doesn't need to be that one specifically, but that one seems to me like it has all the features you would need at a much more reasonable price.
As far as your RAM goes, my point with that is you don't need to buy the Corsair XMS stuff. That RAM is really high quality, but unless you're overclocking (and seeing as you don't know what this means, it's pretty safe to assume you won't be doing it), you don't really need RAM of that quality. Instead, get something more like
this, and save yourself $100. With the motherboard I pointed out, you would still have two slots available to stick more RAM in, as well. You could do up to 4GB in that motherboard.
If you're looking for cases, and you don't want to worry about a separate power supply, I recommend Antec. Their cases are excellent quality, and most of them include power supplies, also made by Antec. Their power supplies are also excellent quality, so they're definitely a good choice for beginners.
Now you don't
need a sound card to have sound. 99% of motherboards now come with a sound card built into it - they call it onboard sound. If you're a total audiophile, then you'll want a better sound card. Also, if you're completely deadset on having the fastest machine out there, no matter how much it will cost you, you'll want a sound card. If neither one of those statements apply to you, however, chances are the onboard sound will be fine for you.
Onboard sound used to be a bare minimum; you would rather just hum all the sounds to yourself than listen to games or music with on onboard card. In recent years, however, the quality has increased drastically. Also, the majority of the time the onboard sound is now 7.1 channels, which means you can have full surround sound without having a sound card. That used to be one of the main reasons to get a card as opposed to using the onboard sound.
Basically, unless you have a very specific reason to get a sound card, stick with the onboard. You'll save some money, and there's an excellent chance you'll never even notice the difference.
I think I got all the questions. If anything's still unclear, feel free to ask. I hope I helped a little.