If you need space, get a Thermaltake Armor case when it's on sale. 5.25" bays all the way down the front of it, BTX and EATX support, and an extra 3-drive cage by the PSU.
Speaking of the PSU, I'll say that 500W-600W is more than enough for nearly anybody as long as you have three or four +12V rails. I personally prefer Thermaltake.
Motherboard, definitely the 650i/680i hybrid chipset - the ASUS P5N32-E SLI Plus is a great example of this. You get 3 PCI Express x16 slots (1 at 8x), a great memory controller, and support for 8GB of RAM, 1.33GHz FSB and quad-core CPUs. Fairly future-proof excepting DDR3 and any new sockets that Intel might come up with.
E6400 if you're overclocking, E6600 if you're not. The Intel price cuts in a couple weeks are going to put the E6700 squarely in the place of the E6600. This is in response to the recent AMD price cuts, which are good but not good enough.
Video card is tougher. If you count eBay, the 7950GX2 is the best single-slot option as it can be easily had for $250-$300 there. Sometimes it's outperformed by the 8800GTS, sometimes it's not, and some people aren't going to upgrade to Vista/DX10 any time soon. If you want to be future-proof right now the 8800GTS 320MB is probably the best choice. ATI/AMD's R600 is going to change things a bit when it comes out, hopefully NVIDIA will lower its prices right now.
RAM - pqi makes dirt-cheap 2GB sticks, in two different lines. I got a pair of the 2GB DDR2-667 CAS4 sticks for $130 apiece. 4GB for $260, and Anandtech claimed to have been able to overclock these same sticks to DDR2-940 at 5-5-5-15. By getting higher-density sticks you allow yourself to use the extra slots when you upgrade your system later on (you did buy a future-proof motherboard, didn't you?).
HDD: Ugh. I can't bring myself to justify buying a Raptor. Faster 15K RPM drives have existed in U320 SCSI (and SAS) for years, and they now cost less per gigabyte than an equivalent WD Raptor, but you'd have to buy a suitable controller card which would even things out (but save your CPU some work!). With 4GB of RAM the HDD isn't going to matter so much unless you power off/reboot your computer more than once every few days. 750GB commands too much of a premium, so 500GB is obviously the sweet spot now. I'd say Hitachi would be the best buy, unless you really really like Western Digital.
Monitor: I still can't bring myself to game on any LCD, let alone one that has a 5:4 or 16:10 aspect ratio! Most games don't support widescreen anyways, or gain any advantage from it. 21" and 22" CRTs have become so cheap now that if you have the desktop space they're great for gaming and work at up to 2048x1536, and you can sit your LCD screen on the desk next to it if you really want one (get a pivoting one and put it in portrait mode, great for chat and the Internet).
| Intel Core 2 Duo E6600 @ 3.0GHz (30C idle temp) | ASUS P5N32-E SLI Plus @ 1.33GHz FSB | 2x2GB pqi TURBO DDR2 @ 667, 4-4-4-12 at 1T | MSI NX7950GX2 500MHz/1200MHz | XCLIO Wind Tunnel fulltower ATX w/2x250mm fans