Custom built baby!!!!
RAM -lots and lots of ram, maybe 1gb of ram (minumum 512mb esp with all the vsts you could be running)
CPU - 500mghz minumum, i did beats for bout 2 yrs with reason with a 100mhz pc, so i think 500mghz could do justice.
HDD - 80gb minimum. with all the samples you will be storing as wav files , you need as much of this as possible. for about $100 you can get a 200gb harddisk on ebay.
GFX card - 8mb will do. you wont need a 256geforce card if u making music.
soundcard - 24bit to get that crisppppppp sound
and you would need at least a cd writer to burn your stuff on cd