I think that $1000 is probably overkill for a gaming PC, unless you want to go high-end.
As far as which case to use with a 450W PSU, any ATX case should be fine as long as you're using an ATX power supply. That's the beauty of industry standards. I think that the Coolermaster Centurion is a nice choice. I used a Centurion 534 to build a PC for my father recently, and it was roomy, cool, relatively quiet and looked good. Plus it was a tool-free case.
G.Skill is a good memory brand, that's what I used on my last two builds. They both worked well with Gigabyte boards. I highly recommend subscribing to the NewEgg newsletter. I see you have 2Gb of DDR2-800 listed for about $70. Last week they advertised a special on a 4GB kit (2x2GB) that is normally around $130, but they sell for $109 and there was a special discount code that took it down to $79 or so. That was a sweet deal. There's usually a coupon code every couple of weeks.
The motherboard is fine, if a bit pricey. If you don't need SATA RAID then you can get a really good P35-based board for $90 or less.
Your CPU will come with a cooler already since it's a retail model, and that should be more than adequate unless you plan on doing extreme overclocking.
Things that I would do:
Drop the dual 8MB cache hard disks and buy a single, larger disk drive. I recommend this one:
That saves you $30. Then drop the CPU cooler. That's another $22 saved. I'd switch to a less expensive mainboard unless you really, really need RAID. I'd go with something like this:
That saves you another $40. Total savings so far, $92. Then swap out the CPU for this one:
You'll get a much faster system for only about $7 more. The only functionality that you're really losing is RAID, but you're getting more storage capacity on a faster hard disk instead.
So then you'll just need to pick a video card. You didn't mention what exactly you were looking for, but an ATI HD 3870 runs about $250 right now, and the nVidia 8800GT is probably $20 more (both for 512 MB versions). If you look around you can get a 512 MB HD 3850 for about $200. I recommend staying at 512 MB if you're playing at higher resolutions (1600x1200, 1680x1050, etc) because you'll get some thrashing from texture swapping with only 512 MB.
Total cost for this very competent gaming machine would be under $800.
