Here are my opinions:
Motherboard - I'd go for Asus motherboard: or the ones thats upgradable
CPU - dunno
RAM - 4 is enough, but trust me you're gonna want to have 8 eventually
GPU - go for the best graphic cards(GPU)
HD - Make sure that the HDD is fast, ur computer performance depends on it
Case - no comment
Optical - no comment
Wireless - yea wireless!!!!, i forgot to put it on my rig lol XDXD
LCD - 22" is the standard size(popular size) atm
Mouse - go for a normal ones first
Mousemat - omg just a cheaaaap one is enough
Headset - this can be more expensive, plus a soundcard is a must !!!
Keyboard - I have Razer Lycosa
but you should go for a cheap one first so you'll have enough money for the rest, unless you have more than enough money...thats a different story