I can scarcely remember the last computer we did not build. I've just built a computer, my mothe, dearest, just built one, and I'm going to do another one for a friend tomorrow. Somebody correct me if I give bad information. So here's the product of my wasted time on research and the advice I've had from some generous sources:
Mainboard or Motherboard:
I've had good success with Soyo Dragon boards. Aside from that, I haven't much to say. You'd just have to check that it supports what you want to put on it. Maybe you want to do RAID or something; that's your call.
Cases:
Pretty is all that matters. And maybe front USB. The power supply is replacable, and if you go for a power supply, you can get a really big one, or you can get a really big one that's good. There's a feature, I think it's called "true power" where the unit supplies more power to that which needs more power. With most power supplies, there's only so much going through any line at any given time. With true power, whatever needs the power gets the power. My Thermaltake says "purepower" on it, and if I'm right, that's another term for it. Maybe it's a brand thing.
Hard Drives:
As good as Seagate is dubbed to be, I've only had problems. I think I've had to send in three or four for replacement, and finally, we missed a warranty. It could have been a bad HD controller on the boards we used with them, but I still wouldn't recommend them. I also have an unjustified hatred for Western Digital. So I don't really recommend them, either.
Memory:
I see no difference in brands, but I'm sure there is one. All I know is that Windows XP doesn't meet full potential until you get to 1 GB of it, and from then on the difference becomes hard to notice. If you run Linux or something, then I don't know what's what.
Optical drives:
CD burners burn out, so get something that matches your case and is cheap. For my purposes, burning speed is pointless. I end up slowing the burn speed to ensure I don't make a coaster anyway. Even cheap ones last if you treat them right, it seems. For $20, you can get a new one. And if you aren't going to already, I'm going to try to convince you to install two optical drives. Probably one CD burner and one dual layer DVD burner. It's the best of all worlds, and it makes copying disks so much easier. One feature I like on a CD burner is that of a play button. With it, I can play a music CD without using any system resources (and it's becoming all too rare.) As far as DVD stuff, I don't look for something with dual burning capability, because you may need the capability down the road. And if cost is no issue....
Floppy drives:
They're all the same except for color and the one I bought. I somehow got one that make NO noise, and yes it's working. I got it off NewEgg, but I think they're sold out, or I'd have two more sitting around. It was a Samsung Model SFD321B/LBL1 if you care to hunt it down elsewhere. So just color. And if anybody told you that you won't need a floppy drive, they're wrong. Something will come up. And even if you never use it, you're only out $10.
Monitors:
Unitl costs come down or you're really cramped for space, I don't see any point to an LCD. For gaming, for word processing, for posting replies to threads, for everything I prefer the look of a CRT. And need I say get a big one? 19" is the smallest I recommend. Another reason I say CRT over LCD. And a person will probalby only need one unless he is running certain software, but you can make that call I'm sure.
Keyboards and Mice:
Yay for the cordless! But if you do go cordless (I just changed some batteries not two minutes ago) make sure the mouse has a charging base. With my stuff, I swap the mouse batteries into the keyboard and recharge the mouse with the base. I was looking at stuff today, and here's what I think looks awesome:
http://www.logitech.com/index.cfm/products/details/US/EN,CRID=2162,CONTENTID=9950. I have the MX 700, and I love it. I think there's bluetooth version out there, too, which is probably better.
Sound Card:
Yeah, I know nothing about this. Your motherboard will probably come with something half decent.
Speakers:
Umm...you must have a subwoofer. You have no other options. Because I said so. No, but in seriousness, you'll probably want some surround sound speakers, which has a subwoofer. Though I've never heard them, I hear only good reviews of the stuff Logitech produces. I'm using some Creative speakers, and they're actually better than one would expect.
Case Mods:
I'm a blue LED fan type-a guy myself. Especially if you have a window on your case. And I know it's a bit excessive, but I went so far as to put blue LED case feet on. And there's a nice blue LED graphics card fan sitting in its box to be installed later today.
Or is that too general? Oh, well, I got to share what I know.