There are all kinds.
If you're going to start from scrath, look at a socket 939 board. MSI, DFI, and Gigabyte are good. Be sure that the mobo you pick out has at least one PCI-E x16 slot. Look for SATA connections, as well as dual-channel memory support.
Buy what you can afford in a CPU. You can always upgrade later.
Buy RAM in sets of two, with a minimum of 512MB. 1 gig would be better (2x512).
Buy what you afford in the way of graphic card(s). The 7800 is the hottest thing going, but it ain't cheap.
Onboard sound works fine, unless you are an audiophile.
Toss in any extras you want and can afford, like a DVD burner, extra hard drive (RAID maybe), extra optical drive, extra fans, aftermarket heatsink and fan, whatever.
If you buy a new case, be sure that you get a decent power supply. The cheap cases come with a PSU, but they are junk. Go with Antec (they have cases with their PSUs), ThermalTake, PC Power, or Fortron. Get 500W or better.