yea for quake 4 you'll want a dual core processor. get like... an opteron, the cheapest one they sell (165 i believe)... or the cheapest X2 (like my 3800+).... or the 4400+ if you don't plan on o/cing.
for a video card you'll want a 7900gt
get a SLI mobo because you'll definitely want to upgrade later on
and like... 2-4gb of ram. i only say this because ive heard rumors of vista taking up 800mb of ram
a decent case mostly for looks but make sure its got like 2 120mm fans or at least 3 80mm fans...
a 500watt or above PSU, preferably antec aspire or thermaltake...
make sure you don't cheap out on a mouse, get something from Logitech or Razr those are good brands (Razr especially if you're a lefty)
any old keyboard (mines a $17 A4Tech) lol
umm.... and may i suggest a 19" CRT? 16x12 = win
if you get a soundcard besides onboard (some people prefer it) get something thats like... audigy 2 ZS by creative labs... those are pretty much flawless.