I pretty much agree with pumeco.  By building (or having someone build) a machine you should save some bucks over trying to find a prebuilt machine.  

I might add consider a full sized gaming case with lots of cooling fans for those long, long renders.  

One thing I did was install dual hard drives running Raid 1 so they are mirroring each other all the time.  I still backup my files occasionally but I like the secure feeling that there are always two copies being saved automatically.  Any slow down time in read/write to HD is imperceptable.  

Yes, purchase as much machine as you can afford because by the time you plug it in there will be something newer and greater out there. 

In the other thread pdrawbridge mentioned rendering multiple cores.  I only have 4 cores running Win XP 64 bit Pro.  When I set Bryce render to priority it will use all 4 cores.  I don't know how it behaves with more cores than that. 

And lastly that GPU based rendering will be a blessing if you ever get into video editing.