Hi sinz and gpepper!
As I´ve said before sinz I always build my own PCs.
The freedome you get is wonderful, but my second choise would be to buy a customised pc where you choose your parts and the shop builds it for you
The specs for the pc I used making the centipede (I have bought a new one a week ago

)
were:
cpu: amd 6000+
ram: 4gig 800mhz corsair (dual channel)
graphics: Nvidia geforce 8800gts 320mb
Psu: 520W corsair
hdd: 1,32tb 7200rpm seagate (3 separate hdds)
and a Intuos 3 A4 tablet.
but after the gpu crashed and burned hehe.. I bought a started building a new pc.
cpu: amd phenom II 955 quad core (3,2mhz)
ram: 8gig 1333mhz ocz (dual channel)
gpu: Ati radeon 4890 1gb
psu:620W corsair
hdd:1,32tb seagate (same as above)
and I hope I´ll be getting my Intuos4 medium in the mail one of these days
a strong gpu is very important for smooth realtime viewports.
a large amount of ram makes you being able to subdivide higher.
now I have to go to work... dont wanna!!!
I´ll think of something more to write tonight when I get home
cheeers