I will give suggestions for every component. You can google or go onto newegg, and pick which one fits your needs the best.
Cases- For this I would recommend getting one of these cases
Antec 300/900,1200
Tempest CM690/HAF932
They have very good cooling, and cable management both things that will help if you ever want to overclock your components
Motherboards- For motherboards I would personally get either the biostar tpower x58 or tpower x58A. What ever motherboard you get make sure that it is I7 compatible. Because of how I7 uses ddr3 instead of ddr2 memory. You'll have to get memory in 3s instead of two. The standard now is to get 6gb of memory instead of just 3. I'd look for a motherboard with a max of 24 GB, and about crossfire and sli the I7 board would be compatible with both.
Processor- I would definitely go with the i7 920 its not the fastest, but you can overclock it to speeds faster then the extreme edition. So in that way it is the best deal. Just make sure to get a good cpu cooler.
* A good thermal past can really help with lower your processor temps and overclocking. I would suggest the OCZ Freeze or IC Diamond. Both very good options
* And for cooling your processor you could use the fan that comes with the i7, but if you want to really push your hardware to the limit.
XIGMATEK Dark Knight-S1283V 120mm Long Life Bearing CPU Cooler
Noctua NH-U12P 120mm SSO CPU Cooler.
Prolimatech Megahalem for Intel Socket 775 / 1366 the best
Thermalright True Black Edition
Ram- Any DDR3 Ram will be good, but ocz ram is suppose to be very good, and they make overclocking very easy with preset profiles so I'd probably get one of those. And I would get a 3 pack of 2gb each stick.
Hard drive- WD is very good so are other companies, but I prefer Western digital. I would try to get a 1.5 TB drive at the least. The price difference between the 1.5 drive and the 1 TB drive is very little. So the 1.5 drive will be the best deal. if you have enough money get the 2 TB drive. Memory is very cheap these days. Oh yeah I'm not sure, but do some research on window vista hard drive limit. I think it can only recognize up too 2TB of memory.
Graphics cards- Okay for graphics cards I would personally get the 275 gtx. In my opinion it is one of the best cards, and it is sold at a very reasonable price. About 200 dollars on newegg from evga. It is only a few fps slower then nvidia's second fastest card the 285. If you wanted to sli I would definitely get two 275 gtx. You'll need a better power supply, but you get more frames then one 295 in lots of games. And you'll save a couple hundred dollars do it. For companies I would either get evga or bfg they have the upgrade program. So if a better card comes out right after you buy a gpu. You can return it pay the difference and get the better card.
Power supply- IDk this always confuses me. I would personally just go with one the corsair power supplies their supposed to be very good. I would recommend the 750HX or 750TX. If you have money get the modular one. If not get the cheaper one. If you want to do three way sli. You'll need one of the hefty 1000+ watt power supply, and it will need 6 pci expresss cables. Two for each card. One thing to remember is don't go cheap on this part. This one of the most important parts you can buy. Read the reviews. Get one that is popular, and that has been tested by other users so you can get the best one.
Operating system- You'll definity want a 64 bit operating system for this computer. Its kind of hard for me to say to you go get vista 64 bit. The reason being windows 7 is coming so soon. It would just be a waste of money. I would wait it out. Do some research, and see if microsoft or retailers are offering any upgrade plan for vista buyers.
Anyways hope this helps