Well with that budget, you have a lot of flexibility. Although I don't think you necessarily need to blow the whole wad on a decent gaming machine.
First off I would go with quad core. Most people will hop on this and say the dual core is better for gaming, and I would have to agree to a certain extent. Here comes the but, but there are a few games in the pipeline (ex Alan Wake) and some already released (ex Supreme Commander) that will utilize quad cores. The trend is more threads, so it just makes sense to plan for the future. You can get a Q6600 for around $230. If you would rather spend a little more and go Penryn, then a Q9400-Q9550 should fit the bill without breaking the bank.
Next motherboard. This is highly dependent. I won't be suggesting a SLI option because I personally don't see the value there. Also nVidia has had some reliability issues with their chipsets lately (SATA), and they tend to be quite hot. So there are really only two options left. If you want to go with a single card with very little possibility of going CrossFire, then pick something with the P45 chipset. If CrossFire is something you definitely want to do, then pick a motherboard with a X48 chipset. Brand, and features should determine what exact motherboard you pick.
Video card is easy. If you have to have the absolute fastest single card, the 4870X2 is the way to go. Again manufacturer will be up to you, base your decision on warranty, bundle and price. If you don't have to have the fastest card money can buy I would get a 4870. The 4850 (at least for me) is too much compromise and with your budget, it's not warranted. Either of these cards provides an upgrade path to CrossFire.
Hard drive is a very subjective topic. Personally I like Seagate and their 5 year warranty. That said I have heard rumors of unreliability concerning their latest generation (11) drives. Since you mentioned movies as one of your prime uses, I would shoot for a 1.5GB drive. I know Seagate has them. As for RAID 0, it's really not needed. Sure it's fast, but for you it's really not worth the hassle or risk.
Sound card is trickier. Most people are satisfied with onboard sound, I'm not one of those. A year ago, I would have wholeheartedly recommended Creative. After the Daniel K fiasco, I won't buy another Creative product. They still supply the best sound solution for gaming though. I am intrigued with ASUS's Sonar line. They simulate EAX 4 as well, though it isn't a flawless emulation from what I've read.
Now although you didn't mention it, it's a common mistake by first time builders. A PSU is nearly the most critical part of your computer, and the most overlooked. Picking an actual model requires the you finalize your build as far as GPU(s) and to a less extent CPU. These two components are going to draw the most power from your supply. Stick with respected manufacturers. The likes of PC Power & Cooling, Enermax, and Corsair's high end supplies. A bare minimum for the low end of my suggestions would be 600-650W. If you shoot for a 4870X2 or CrossFire 4870's, I would go above 700W. Remember to give yourself enough headroom to upgrade later.
Also a case is pretty important. You want something that has plenty of room, air flow, and isn't too noisy. There are a lot of options out there, and picking a case can be highly aesthetic.
Hope that gets you started. Update this post with your suggestions as well as any components you decide on so that we can critique them for you and steer you clear of any pitfalls.