are you looking for a desktop or laptop? are you looking for new or used? used can be found on ebay, simply type in "gamer" and put in a price
if you want to buy a new pc ... do you know how to build a pc? if not, buy one pre-built from ebay. but if you know how to build pc's... hit up:
Newegg
Tigerdirect
Geeks
i'd say go ahead and invest in a good vid card (for cataclysm) - make sure it supports directx10 - this will be one of the most expensive pieces. you will want at least 4gb of ddr3 (if not 8, 12, 16gb). Also, please give us a "price" on what you want to pay? are you looking for a super cheap $200 pc, or can you pay $1500 for a pc?
what features are you looking for exactly?
a pc i recently bought just for my bots is below
$365 shipped (i messaged him after auction ended since no on bid on it)
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=330392086784&ssPageName=STRK:MEWAX:IT
Heres what i would recommend (i shop newegg for new stuff)
Intel i7/mobo combo
16gb DDR 3 (in time w/ ur fsb of cpu -- dual/quad channel ram)
1GB pci-e vid card
500-800w power supply
500gb-1TB hdd or two
A "cooler master" box (maybe combo w/ power supply) or equiv. something that has good air flow
a very, very good cooler for your cpu
make sure you have peripherals (mouse, keyboard, monitor, speakers/sound, etc)
optical drive (dvd/hd/b-ray)
you can probably run 12 bots on this, or more
windows ... u can use a legal version you have. torrents can get you free stuff, but that's illegal to do, and i dont think i can advise you how to do that on this forum
for bare minimum (cheap):
duo-core cpu + mobo combo
4gb ddr2 or ddr3 (matched to fsb of cpu)
box + power supply (400w+)
512mb+vid card
200gb+ hdd
cooler for cpu (if not included on combo)
basic dvd drive
if you decide to try and build your own pc... its not very hard, you just need to check a few things:
case - it should open to 1 side (some have the whole side, top, and other side as 1 pulloff unit)
mobo fits in and needs at least 4 screws to hold it down (you'll see bolt holes inside the case)
the cpu you fit into socket on mobo and LOCK IT DOWN via a lever or turnkey (laptops have turnkeys). make sure the cpu is seated properly before you lock it, or it will damage the pins
2nd note: you will need thermal paste (included w/ cpu) or Arctic silver (for power users/gamers/over clockers).
then you seat the huge cpu heatsink/fan onto your cpu and
PLUG IT IN
the ram you seat into the long ram extension bays, seat them evenly
the optical drive is 5.25" (im sure you know where it goes) make sure you use at least 2 screws, if not 4, to screw it into its slot
the hdd(s) (hard drive) use 4 screws (minimum 3) and it goes in the smaller bay (3.5")
the power supply is usually screwed in with 4 screws to the case (backside). there will be 1 big plug that goes into mobo, usually a 2nd'ary plug (4 pin square) that connects near the cpu, and some vid cards have various power cords that need to go in them. also the optical drive and hdd both need their own 12v connections (4 pin, long)
the vid card will need to be seated in the pci-e slot (usually only 1-2 slots for pci-e on a mobo)
when plugging up cords to the rear, use common sense and they all fit easily.
i hope this helps