well.. in that case you may consider for a economy class
p) sound card (e.g.Creative Vibra 128, ~$30) or if you want more *sound* from your computer then get an Audigi
and you don't have to worry much about the LAN bit because it's built into most motherboards these days or you could get one for ~$15. Ooh.. yes, if you could be satisified by onboard sound, you don't even have to get a sound card
(you can have 5.1 even with onboard sound)
And motherboard.. if you are not a computer junkie who always upgrade and can wait for 2-3 years, you can probably settle with motherboards in the $100-$130 range, also, check out what CPU you want.. if you want a cheaper CPU, an Athlon is definitely your choice, personally I would recommand the XP 2600 chip (it's always better to get the semi-latest chip because they are not very expensive compared to the top one and you still get good performance). If you want a P4 you'll be looking at an extra 100 bucks compared to the XP 2600.. but I don't think that would be necessary in your case
hard disk... go for a 80GB one, the difference (in price) between a 40GB one and a 80GB one is only about $10-$20.. if you want better performance, pay an extra ~$30 for 8MB of buffer
If there's not much gaming, 256MB (DDR 2700, can go for DDR 3200 if you want to) would be sufficient and then all you'll need is a normal graphics card. Here you are probably looking at the FX5200/Radeon9200.. if you are not sure about what graphics card to get, it's always better to get the mid-range ones (i.e. FX5600/Radeon9600 Pro) and that would cost you from $90-$140 or $160-$300 respectively (that depends on the brand.. but the cheaper ones also work really well)
assume you want a DVD combo drive.. that would be ~$100.. just add them up together
post again if you need more help
p.s. if you need a specific spec. and price, you'll have to tell us EXACTLY what you want
including whether you'll be gaming or not.. etc..
Note: those posted above are just personal recommandations