Username or EmailPassword
My belief is that you should certainly buy your first computer from a retailer. After that you should upgrade yourself.
For you second computer, I would suggest having a friend that already build his own succesfully help you. That is because a lot of things can go wrong and cost you more, if you are not careful.
Now as what to buy... Not the top, and not the lower end of the tech spectrum. Just buy what you can aford and have some money left for extras (like a nice looking speaker set :-) ).
My suggestion as what to buy (If you want enough power, but you are not an extreme player):
FULL TOWER ATX Case
AMD Athlon 2000+ XP
Gigabyte Mainboard (DDR,Firewire,Serial Ata,USB 2, Audio, Network card, AGP 8X, NO VGA onboard)
512 Mb Ram DDR 333
80Gb HDD Western Digital or IBM
GeForce 2 400 MX TV/Out 64mb ram
Acer CDR-RW 48X
Optical mouse (very accurate, can't do without one now.)
As for a monitor, I leave it up to you because this is too personal of a choice to suggest. However don't buy anything below 17 inches.
If you find the right dealers, this system should cost less than 1000 dollars. The only parts above 100$ are the motherboard, the Hard disk and the monitor.
If you need something with more kick (for GAMES) to it then a P4 2.6Ghz and a GeForce 4 FX 128Mb or Radeon 9700Pro is what you want.
NOTE: I have worked on both high end Athlon and P4. P4 only provide better Gaming for the average user/power gamer. The only other reason to consider a P4 would be if you use something like 3D Studio Max.