I used to be able to crack out system recommendations easier than I could wake up in the morning. Alas, the market has changed greatly.
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I'll start w/ mobo's:
Nowadays it really depends on whether or not you want to rock DDR2 or DDR3 memory. DDR3 memory is not ready for mainstream, yet, so I suggest saving your money and rocking a stable DDR2 board.
The best DDR2 boards out there right now (as favored at Tom's Hardware and the general intarwebs) are the Gigabyte X48-DQ6 and the DFI Lanparty LT X48-T2R. I've built systems with both DFI and Gigabyte boards. From my experience Gigabyte boards are rock solid stable regardless of what you throw at them. The DFI boards can be a bit finnicky, but the performance potential is higher - especially for overclocking. Hands down the DFI boards are my favorite boards to OC on. If you plan on OCing, I'd suggest DFI; if you don't, I'd go with the Gigabyte.
If you want DDR3, the best few boards seem to be the Asus P5E3 Premium and the Intel DX48BT2
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Graphics Card: Picking a GPU is one of the easier tasks, considering the market turns over every 6 months. Only hard bit is the company. As far as nVidia goes, EVGA is the way to go. End of story.
Their cards are right next to the top of the line performance wise, their customer service is unrivaled and usually intelligent, and their stepup program is a big seller in the GPU industry.
If you go ATi, I can't help you much there. I haven't used an ATi card in over 6 years, so I'm not up to speed in that department.
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Memory: As I already said, DDR3 costs way too much and delivers far too little for the price increase. Stick with DDR2 unless you really, really want DDR3.
DDR2-800 or DDR2-1066 seem to be the best options right now. Company/make will make very little difference overall, but I'd stick with Corsair or OCZ for the Gigabyte board and then g.Skill for the DFI board.
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HD: I've tried who knows how many different RAID and single drive setups with 10k, 15k, and 7k drives. In the end, I've come to like the following:
OS installed on a small (35gb) raptor with everything big and clunky stored on a different, larger SATA drive. 10k raptor for the small drive, and then something like this
http://www.newegg.com/Product/Product.aspx?Item=N82E16822148262 for the larger SATA. (That's my favorite bang-for-the-buck drive, BTW)
If you want maximum speed, run 2 raptors in RAID 0. Beware though, while this may be hellishly fast, it's also hellishly unstable. If I crashed even once the whole partition was likely to be FUBARed. (I got really, really good at reinstalling windows)
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OS: Up to you really.
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Case: EFFFF I really, really, really wish I could remember the name of the watercooling company I had always wanted to go through. I became obsessed with noise for about 3 months last year and did an ungodly amount of research into water cooling. I'll text a few friends and see if I can get that name before the end of the day.
Koolance is an okay, no-nonsense setup, but there are a few better. I personally wanted to run XYZ from aforementioned mystery company in a Thermaltake case. If you do run a 3rd party water setup, I'd sport a thermaltake case. By far my favorite cases.
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PSU: Don't look at the Wattage so much as you do the Amperage. If you post the GPU you want to get, I can make some good recommendations for PSUs. Most people get duped into buying 1500W PSUs which are shitty and just make their ePeen feel bigger. I've never had to put more than a 750W PSU into a system and I've never had any problems.
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Hopefully that answers some questions. I know I haven't done this in a while, so things may not be quite as solid as they used to be.
