do you have a system and psu you are willing to run 24/7?
do you already have an amd (juniper, cypress, barts, cayman)video card?
do you have enough disposable money to live without $1000 for 3+ months?
if the answer is no to the questions above, you probably shouldnt think about mining. otherwise .....
cost of system
cpu: cheapest will do.
mb: multiple pcie slots req'd (x16 and x1 x1 will do.)
ram: minimum will do
psu: 800+ watt high efficiency with at least 4 pcie plugs, 6 is better.
dvd/external drive to install os: optional
os: free if linux (way more configuring to get mining) or cost of windows lic.
cost of cards
any 58xx/68xx/69xx card will do, but at $200-360 per card the return ratio goes way down. that is why 5830 @ $110 is the bargain.
3 cards is probably minimum now. if you can even find them for sale.
cost of electricity.
depends on your area and weather(ac over summer)
--take total hash rate of video cards
https://en.bitcoin.it/wiki/Mining_hardware_comparison
--apply to calc
http://www.alloscomp.com/bitcoin/calculator.php
--assume difficulty will increase and multiply weekly profit by .66 to get next weeks profit. do same for following week.
go online and price out the bolded items. find out return rate and diminishing profit schedule. find break even point in terms of weeks/months.
Decide if return rate is acceptable over time period you are willing to mine.