I agree with Adam. The specs you gave would have been great about four years ago, but you shouldn’t settle for anything less than an i5 or i7 chip with as much RAM as you can afford (I would suggest 8GB at least).
Mac Pros are nice, but they are still waaaayyyyy too expensive. I also agree that a beefy GeForce card will suffice beyond a Quadro unless you want to drop 1-3 thousand on a new video card (which you won’t really need unless you’re running Massive or some comparable program). Furthermore, I would highly suggest an Nvidia card, as many Adobe products are “optimized” to run on their hardware. Also, in looking towards the future, there may be some nice CUDA development being done on a consumer level in years to come.
Regarding OS, if you’re leaning towards a PC, you definitely want to go with Win7. I’ve been running it for around a year now and have had no major problems running C4D and the Adobe suite.
Finally, I would suggest buying your components from NewEgg, as they tend to have the best price/customer service, but that’s just my opinion…
Hope this helps.
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