Power supply: arguably the most crucial component of the system - converts the AC mains voltage to DC voltages that the computer's parts need (3.3V, 5V, and 12V are the key ones, -5V and -12V are also supplied but rarely used) and hopefully isolates the computer from the rest of your house's loads in terms of "noise". Power supplies also provide an exhaust fan, which is often the main one in the PC. They're rated in terms of maximum power (watts), which can be misleading because of the fact that each voltage path usually has a different maximum. For a modern full-size system, anything less than a 250W supply is risky, and realistically it's not a bad idea to get a 350W or 400W supply. PC power supplies are switching (as opposed to linear) supplies, which means that they only draw power to satisfy the demand from the load - this means that e.g. a 400W supply is not burning 400W all the time, but rather whatever is being burned by the system (plus 10-15% or so power loss from resistive components in the supply). Preferred brands are Antec, Vantec, Enermax, and Thermaltake, among others.
Do not get a cheap no-name PSU - bad power is an absolute bitch to diagnose because any component can behave as though it's faulty if it's not getting a good supply, and problems may only show up under relatively uncommon circumstances. Often when a system hard locks or resets out of nowhere it's because there was a power failure just large enough to hose the system.
Motherboards: The job of the motherboard is - put simply - to allow everything to talk to everything else. The main measures of a motherboard boil down to what/how many things it can connect and how fast it can let them talk. For practical intents, the bulk of the motherboard's work is done by two chips, traditionally known as the north bridge and the south bridge. These are typically sold together by chip manufacturers as the "chipset", but different combinations are sometimes used. Pentium chipsets are dominated by Intel (whose chipsets I'm not familair enough with to summarize). Athlon chipsets are currently dominated by the VIA KT400A and NVidia NForce2.
Key north bridge features are:
System Bus (sometimes called a "front side" bus, to distinguish it from a "back side" bus which is a bus dedicated to cache memory): connects the CPU to the rest of the system. Basically this will support an Intel (Pentium/VIA) or DEC (Athlon) interface standard at particular speeds. Pentium 3 bus speeds are 100MHz and 133MHz. Pentium 4 bus speeds are 100MHz, 133MHz, and 200MHz QDR (labeled 400MHz, 533MHz, and 800MHz respectively). Athlon bus speeds are 100MHz, 166MHz, and 200MHz DDR (labeled 200/333/400MHz).
DRAM Interface: connects the RAM to the rest of the system. Much like the CPU interface the main features are the interface standard (SDRAM, DDR SDRAM, RDRAM, etc.) and interface speed. Recently Intel has somwehat favored RDRAM while everyone else has favored DDR SDRAM. The current high end for DDR SDRAM is "DDR400" (200MHz DDR, also called PC3200), while the current high end for RDRAM is "PC1033" (533MHz DDR).
AGP Interface: Connects the graphics card to the rest of the system. AGP 8X is the current high end speed standard, and an extension called "AGP Pro" provides extra supply connections for certiain power-hungry cards.
South bridge Interface: Not much to say about this. In older chipsets this is done with PCI, in newer ones it's using a proprietary or semi-proprietary dedicated bus.
Key south bridge features:
ATA controller: Talks to ATA/IDE hard drives. The current high end standard is ATA133. Most south bridges support 2 ATA channels, each of which can support 2 drives. Many recent boards however supplement this with a dedicated ATA chip. Chipsets supporting Serial ATA will probably arrive soon (if they haven't already). Serial ATA actually has very little to do with ATA except that it's intended to replace the hardware side of it; it uses a 4-wire cable instead of the rather unwieldy 40/80-wire cables used by ATA.
High-speed serial I/O: USB 1.x, USB 2.0, or Firewire. USB 1.x is the most popular standard for most peripherals. Firewire has a substantial niche in external drives and digital video equipment, but USB 2.0 is taking away some of its dominance for external drives.
Ethernet: Connect your computer to a network. Requires an additional chip to do the voltage conversion / signal encoding as well as the connector, so not all boards with a particular chipset will actually have Ethernet support.
Audio: Usually pretty barebones compared to a good card but serviceable for anything that's not demanding (e.g. realtime audio composition, MIDI synthesis, "environmental effects" in games). Like Ethernet, requires another chip/connector to actually work, so not all boards will make use of this feature.
Legacy I/O: All the good stuff you expect to be on a standard PC - floppy interface, serial ports, parallel port, and PS/2 keyboard/mouse ports.
PCI: Lets you plug in PCI cards.
It's worth noting at this point that Athlon 64, due out later this year, will change this picture quite a bit. It will move the DRAM interface into the processor itself and will have a sort of "southbridge interface" built in for interfacing to AGP/PCI bridges and I/O chips. Basically it will shatter the concept of the system bus as we know it today. See you in September.
Video cards: What do all those damn numbers mean?
Your guess is as good as mine, but don't go getting a $400+ on a video card. They are not
that much better.
-Drives: I know a decent ammount about these, just mainly wanna know what brand to get....plextor?
Hard drives:
Every hard drive manufacturer has dark spots on its record, but the current overall price/performance/reputation winner seems to be the Western Digital "Special Edition" drives.
CD-R/RW: Plextor and/or Lite-On. I hear Plextor just came out with a drive that can record 1GB/111 minutes on standard media, readable by most standard drives...
Recordable DVD: Pioneer if you're willing to do without DVD+R, Sony if you want to go all the way and support both DVD-R (pronounced "dash-R" not "minus-R") and DVD+R.