Each to thier own and whatnot, but for a cheap Amithlon machine you cant go wrong with a socket775 mobo using ich7/8/9 (p35/p45/g31/g33/some g41) + a cheap core2duo. Im using a e5300 in my amithlon box (clocked at 3.86ghz (up from 2.6ghz, on standard cooling)). Ive yet to see an e5xxx cpu that didnt at least clock to use a 266mhz bus (up from 200) (basically add 1/3rd clock speed as a guarantee, usually more). Mine does go higher, but beyond 3.86ghz it starts to add a few extra degrees. Great value for money, and to my knowledge the only semi recent machines that have been properly tested.
As for network cards, off the top of my head:
ne2k compatible (rtl8029 is common), rtl8139 (via openpci), and some 3com cards (I cant be more specific here sorry) are the cards Ive used. Sound wise its a little limited, but ac97 that utilises via 686a/b chipset combo, sb128, and sblive are the most common options.
Again, feel free to ask for any more info or help if you need/want it.