I run 1 hlds server on gearbox
i spawn about 25 or 30 zombies to slaughter and the cpu on my 2.8 p4 never goes above 15%
here some specs

server specs "for those who care"
p4 2.8Ghz
512 MB DDR333
ASUS P4S8X-X mainboard
gigabit lan
nvidia g force 4 ti4600 8x agp 128MB ddr
win2kpro sp4

my client pc specs "for those who realy care"
celeron 2 667 Mhz
128 Mb pc133
SOYO SY-7IWM/l V1.0 mainboard
10/100 realtek
3dfx VOODOO 3 2000 video "no voodoo is not dead I say -16bpp works perfectly! and of course I use half-life's 3dfx mini opgl driver" :p
windows 98 se

servers bandwith as follows

2003Kb down
257Kb up