It depends in you usage. If it is for gaming/video editing, you really need discrete GPU.
But, if you just browse the net or something like that, onboard graphics is enough.
In $50-$75 GPU, I recommend getting HD 6670.
It can handle some of the current games in medium. Some are even in high settings.
If you can add $30, pick the HD 7750.
Now, we are the same!
My other PC has:
-- P4 2.66Ghz.
-- nVidia FX 5500.
-- 160 IDE HDD.
-- 1GB DDR 400.
Well, almost the same.