Power supply. "dirty" power can lead to poor quality burns, inconsistent scans, and can also burn out components (like DVD-RW drives). The ones from OCZ, Fortron/Sparkle, Enermax, Seasonic, or Antec are all good. If you're going to spend money, do yourself a huge favour and don't cheap out on power. Cost and looks have nothing to do with whether a PSU is any good or not. And you don't need huge wattage to run even multiple DVD burners; 450W and up should be fine now and for the future, but only from the manufacturers listed above because their power ratings are accurate, unlike the no-names.
Motherboard - there is no "best" or "wrong" one; the only question is do the supplied features meet your specific needs? Do you need CPU overclocking? Extreme overclocking or just a little? Firewire? Dual PCI-e x16 slots? 10 USB ports? Onboard graphics & sound? Lots of PCI slots? For example, a DFI Lanparty UT nF4-Ultra-D is probably the best overclocker (for AMD CPUs) because of BIOS options, but the MSI K8NGM2-FID makes for a better Home Theatre PC because of the good onboard video & 8.1ch sound. Asus makes high-quality boards with very stable BIOS settings but can be a little expensive for the more full-featured ones; you might want to look at their lower-cost "ASRock" (micro ATX) models. If you're thinking of doing stuff like video captures or high-end sound, you might want to avoid a micro-ATX motherboard and go with one that has a lot of PCI/PCI-e x1/x4/x16 slots just for the flexibility of adding those kinds of cards later.
CPU. Frankly, if you're not playing games, just about anything current will do, even a 2Ghz Celeron. It doesn't take much CPU power at all to burn a DVD, not even at 16x. Dual-core CPUs would be wasted in a burning environment because the true bottleneck with burning is the speed of the I/O, and not the CPU. The WD Raptor hard disc you like would do more to help your DVD burning than any CPU would. Still, if you want "the best possible", check out the prices on the AMD Athlon FX57 or FX60.
Memory. Again, if you don't have gaming/overclocking requirements, just about anything from Mushkin, OCZ, Corsair, G.Skill, or even Kingston would be fine. If you go with an Intel CPU and want to overclock it, the expensive low-latency RAM might show a performance difference (although not in DVD burning). If you go AMD, the on-die memory controller means that just about any quality manufacturer (see above) value RAM offering will do, without regard to timings. As far as quantity goes, I've yet to see where my 1GB wasn't enough. Games, as always, are another story - the more the better. To ensure best performance, buy either 2x512MB or 2x1GB dual-channel RAM sold as a matched pair. Matched pairs are tested together at the factory.
Hard Disc. The 150GB WD Raptor is just about the fastest one out there, and a decent size. You might want to consider a second one, not as fast, but with massive storage for DVDs, like the Seagate 7200.10 SATA-II models that go up to 750GB. When it comes to DVD burning, capacity trumps hard disc speed, and it's here you should be looking to spend money. Make absolutely sure that the SATA-II drive you get is, in fact, fully compatible with your motherboard's chipset. I've heard stories about nForce-chipset motherboards not liking Maxtor or Hitachi SATA-II drives.
Case. I have a 6 year old Pentium-II case, and the CPU temperatures inside range between 32C (CPU at idle) to 42C (CPU at 100% load). And the Heatsink/fan is the one that came with the AMD CPU. No chassis fans, no HD coolers, no exotic water-cooled pipe systems. I'd go for looks, but I guess your main consideration is how many 5Â¼" drive bays it has. Antec has a series of nice-looking cases with innovative features like sideways sliding drive cages, and rubber grommets to keep noise & vibration down. Some even have air filters to keep the insides dust-free.
Video card. Games is all that matters because the only difference between an onboard solution and an expensive PCI-e card is 3D performance. If you want to play relatively mild games like SimCity4 or The Sims II, look for motherboards that have an onboard GeForce 6150, like ASUS A8N-VM CSM, MSI K8NGM2-FID, GIGABYTE GA-K8N51PVMT-9, or Foxconn 6150K8MA-8EKRS. If you find later that you want better game performance, each of those has a PCI-e x16 slot that you can put a more expensive card into later. Note that onboard video often can only supply one analog signal at a time; that is, you can choose to output to (HD)TV or VGA, but not both at the same time.