I would guess it is your PSU being not good enough.
What PC weaker than yours are they using to max it out with?
My brother has got:
Intel Core 2 Quad 2.6Ghz CPU
ATI HD 4870 2GB
Corsair HX or CX 850W PSU
ASUS Mobo (not sure of exact model)
2GBx4 ddr 3 RAm
Acer 20" screen
He doesnt overclock or nothing.
I use 24" Samsung. 1920x1080.