1792mb of memory is a gimmick. 896 mb to 1gig of memory is more than enough memory for anything your card needs. The newest games are often designed for PC's and consoles which do not have such insane graphics cards so they are optimized to not use up so much memory. By the time games start using over 1 gig of memory your graphics card will be too slow anyways. If you buy that much memory on your card you're just throwing your money away.
I don't know the length of the GTX 275 is but I did see them priced very well on Newegg. I'm pretty sure that if you get a decent mid tower case you'll be fine though. The reason I and others like EVGA so much is that their cards are reliable (never heard of someone getting a lemon EVGA card) and their support is great.
When you spend $150-200 or more on a graphics card that's going to be highly stressed and get very hot you want to be sure you'll be covered if it goes bad. While I like companies like Gigabyte (I've had a few of their motherboards over the years) their support is so-so. I've owned an XFX card that never had any issues but I've heard bad things about XFX's support.
EVGA is usually priced competitively and they have great service so why not go with them? Plus they have a 90-day step-up program where you can get your card upgraded for free if a better card comes out that's priced the same or lower within 90 days of purchasing your card. That's awesome.
Here's a great EVGA GTX 275 for $200 after rebate:
http://www.newegg.com/Product/Produc...82E16814130475
As for a case, the best case I ever owned hands-down was the Antec P180. It's heavy but it's sturdy, absolutely sound-proof and has pop-out lint screens in the front to keep everything inside perfectly clean. No sharp edges and trays slide right out for easy swapping of components. Plus it's very easy to keep your wiring nice and tidy for better airflow. Antec has updated the P180 with the P183 which is even better. Unfortunately for me they don't make a Lanbox style case in that series that fits big graphics cards so my latest computer isn't an Antec case (but it probably would have been if they did!). It's on the pricey side but trust me you get what you pay for:
http://www.newegg.com/Product/Produc...82E16811129061
Unfortunately this case doesn't get free shipping from Newegg. I would stop by a local computer store to see if they have it, it's a pretty popular case and I've seen it at all the local Fry's stores.
Have you gotten a power supply yet? I think 700 watts is as low as you want to get if you're going to be running lots of high end stuff.