On top of what danbuscus said you also have to take into account what your Internet can cope with. Go to http://www.canihostaminecraftserver.com
and follow the instructions there.
So you are limited by RAM (4GB for you after you take off what your system will use for everything else), how long you can keep your pc on for, and the Internet. You are also limited by your cpu which will need to be at least 4, preferably 6 cores and be 3GHz plus.
Other than getting a monster pc and fibre optics, The best thing to do is probably to pay to get a server hosted. I use easy network solutions http://www.ens-ltd.com/billing/aff.php?aff=104
who host in the UK and USA. For 30 slots you'll probably need around 3GB which is £12 a month I believe.