My two cents.
Remember the sit you are in now? buying a pre-built forces you to limit your upgrades in the future, but they are usually cheaper and come with the os and other software, some though is mostly trialware and junk.
A Personal PC (yes redundent choice of words) will allow you to tailor to your wants and needs, but requires some knowledge of PC building (easy enough to learn and we would be glad to help out on any questions you might have) and more expensive than a pre-built. You will need an
OEM of the OS which can run you over $100 ( I know MS raised thier price, but unknown how much),
a motherboard, your first prize posession and first priority on the upgradability scale,
A video card,
a sound card ,
a Hard drive, a floppy drive (I use it for boot purposes for old machines being a pc tech, but with boot cd capabilities on a lot of MBs and thumb drives that can be used for tansferring, which holds more and around the same price in most cases, making the fdd useless),
an optical drive (cd-rom, dvd-rom, cd-rw... get it? I recomend an everyday cd-rom for stupid protection problems, see KOTOR intall thread in same forum to get my drift, and a writer of some sort. Aim for a DVD writer cause it also writes cds, but they can get pricey),
RAM,
CPU,
CPU fan, and a
case (some cases now in days don't have a
power supply, so be ready for that. A PS is your your second prize posession for it provides wattage to everything you put in the PC. Min 400 watt.).
A network card is an option, but recommended. 10/100 min. Modem too is optional, too. These two depends on the Internet connection you choose. Highspeed, network card. Dial-up, modem. Not planning on an internet connection (you wouldn't be talking to us or reading this if you had

) forget both until you need them.
Feel free to add any other essentials guys if I forgot something somewhere.
As you see, it's tit-for-tat and really depends on your choice really (less hassle, less upgradability or more hassle, more upgradability). I personally recommend the BYO route because you don't have to really come up with a lump sum, just buy the pieces as you have the money. But if you are impatient like me, then you'll need a lump sum, and will be, as I said, more expensive.