Come back to us in a month - hardware prices can fluctuate quite a lot, plus new stuff comes out. If we specced out a build for you now we'd probably have to change several things by the time you buy.
1. Overclockers, Scan, eBuyer, they're all good.
2. Ivy Bridge (new enthusiast-level Intel CPUs) are coming in january or february, some time around then. If you're gaming though, the current Sandy Bridge CPUs will be plenty.
3. Personally I'd go for Nvidia, they're slightly pricier currently but I tend to prefer Nvidia drivers (things like program-specific forced anti-aliasing is a lot easier with Nvidia). This is something that we can give you a better answer on in a month.
4. 8GB is plenty for gaming. The only reason to be going with more than 8GB is stuff like professional image/video editing and using VMs. Tbh 4GB is enough but 8GB keeps you safe until you next upgrade.
5. There are loads of good cases around nowadays compared to when 775 was newish. Also something I can better recommend come October.
6. We can help you with that easily
Just get a decent-sized case with several large fans, plus a decent CPU cooler, and you're fine.
SSD prices in particular are going down all the time, so that's definitely something better to recommend nearer the time.