My advice...
If the only thing you do that's really taxing is gaming, an AMD is fine, in fact they work great for pretty much anything.
I like AMD, but if you do anything else on a regular basis that needs some serious grunt, go with an Intel I5. If you want raw power, Intel wins. Period. At worst you spend a bit extra, but other than that, you really cannot go wrong with Intel. As for I5 vs I7, the only benefit of an I7 really is hyper threading, which games don't really take advantage of. While AMD can be cheaper, when you compare them power to dollars, they run about even with Intel, they aren't quite the bargain many immediately think they are, in fact the real savings often come from the cheaper board they use.
Board, I recommend ASROCK or Asus. I have had very few problems with them, however, I've been a bit less happy with Asus lately.
On the other hand, if you want to run OSX on occasion, Gigabyte is a better option. I just get tired of chasing oddball drivers and bios and such that they like to use. Oh, and go MATX.
Memory,
You only need 8 gigs. I have 16 and it's VERY hard to use more than 8. Don't get me wrong, if I built another desktop today, it would have 16, maybe even 32, but I always over build. It's easy to add more later if you need it. 8 is perfect, any more takes money away from the rest of your system. Also, don't buy into the huge heatsink, overclocking memory. Basic memory from a good brand will work just fine, you also don't need the fastest. DDR3 1600 (pc3 12800) is perfectly fine, DDR3 1866 (PC3 14900) is better, but overkill and over priced. I prefer lower latency and more memory over raw speed and less memory.
Case and power supply
This is where you are a bit in trouble. You want small, with a full size graphics card. You wont get a combo, so...
Get a 500 watt PSU, anything with a good name brand will work. Corsair obviously is a good one, but you will pay more. An Antec is just fine too. 500 watts is enough that any good brand in that range should be fine.
The case...
The Fractal Design 1000 is actually darn nice for the money at only $30. I have a few of them and they are a bargain. Power Spec makes a similar case for about $25, I have a few of them too. Both have good airflow and are just hard to beat.
If you want something a bit nicer... SilverStone SST-PS07B is about as small as you can get and still use a full size graphics card and standard PSU without looking like a shoebox. These run $80-$100.
Basically, this leaves you with your choice of drives... I like SSD, but it may not be in your budget and the video card. I would spec the drive you want, and then buy the best video card you can with what's left. Any video card over $200 will play almost anything out at decent resolution and quality, you don't need a $500 card these days and much above $300 or so and you start getting less and less for your money.