CPU: At least 2.5ghz p4 or AMD
RAM: 512mb or greater.
Video: ATI or NVidia 64 or 128mb card. nothing fancy
OS: XP pro, unless your work calls for a certain Linux version.
HDD: 80gb or higher, SATA
CDrom: DVDrw/cdrw drive
LAN: if you buy from dell or gateway, etc, they will have an onboard LAN right on the motherboard. I'd stick with wired, unless you get a laptop, then get a centrino chip that has wireless built into it.
sound: Whatever the basic sound card option is, should work.
If you're going to upgrade in the future, get a whole lot of ram, top of the line sound card ($200 or so), most expensive ati or nvidia card ($400), fast sata HDD, and whatever the top of the line processor is.
For a basic computer that you've described, you'll spend about $500 with a cheapo monitor. If you get into heavy duty gaming, $4,000 is a good budget to get into. You can "build" your computer at one of the retailer's websites to meet your price and performance needs. good luck and let us know what you ended up with.
I personally build my computers, so they don't have a service plan. I think its a waste of money to buy a service plan. just my $2000/100000.