I would never ever spend more then $1000-$1500 on any single computer. This is because no matter what you buy today, in 1 year you can expect that there will be something faster, cheaper and more shiny and your current rig will be slow and crappy in comparison. Buying computers to 'last' is a suckers game.

And n-thing the 'get better software' suggestion. The fastest hardware you can buy will still be hamstrung by poser/firefly whereas even moderately powerful hardware on one of the better 3d apps will outperform anything you can do in poser. And i don't even mean super expensive apps like Maya or MAX or whatever. Middle end apps like modo, lightwave, even vue are all reasonably priced and run circles around poser in terms of speed and quality.