first of all, how is it 768mb ram... oh! 512+256. ok, heres what i would say.
AMD 64 processor, need to know what number it is(3200, 3300, something like that) so i can look up the frequency (ghz).
the ram is ok, but if youre gaming, please list what games, because it will be inadequate for some.
120gb hd is fine, dvd and cd stuff is fine.
please ask what kind of GeForce video card it is, because that can vary from a $20 card to a $600 card
a built in sound card is not custom, as far as i know its fairly common, just built into the mobo.
unless hes some really crazy technophile who makes his own motherboards, its not custom.
if you can get more specific with the monitor, the processor, and the video card, i could give you a price.
how big is the monitor? like 15" or like 19" or what
P.S. Welcome to TF