as W&W said theres really no excuse nowadays not to get 16 gigs of RAM. ive seen decent 16GB kits as low as $70.

buy the processor you can afford.  for rendering and multitasking, number of cores are more important than core speed. the older AMD Phenom II X6s still offer a tremendous bang for your buck in a pure work machine. ive seen 1090Ts on Newegg for as low as ~$100 now. I have one overclocked to 4ghz and its a workhorse. just make sure you get a good power supply and preferably a 125W TDP board with 8-phase power (or if not, then at least an ASUS board that has overcurrent protection). get MOSFET coolers if your board doesnt come with them. stay the hell away from MSI and all budget boards when building with the Phenom X4s and X6s.

graphics card is not really important for Poser and can be upgraded later on. yes, someone will come and point out that it makes a difference in the workspace preview. yeah, you can turn up more of the bells and whistles in the workspace, but in the end its pretty meaningless. i can work just as well on my old machine with a 7600GT as i can on my new one with a GTX 580 -- at a fraction of the cost and power consumption.

64-bit OS is also a must.

a quality power supply is also a must. DO NOT cheap out on a power supply. you should be getting a quality unit from Seasonic, PC Power & Cooling, Enermax or Antec.  Corsair is also OK, but i no longer buy them because i have two that have really annoying capacitor whine even at only 50% of their rated load.

so in summary, id recommend a quality PSU, 16GB DDR3 kit, mid-range ASUS AM3 board paired with a Phenom II X6, Windows 7 64-bit and either use onboard video or get an entry level graphics card. you can easily build this for under $500 and itll be hard to beat for a budget work machine.