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AutoCad 2D is not tremendously demanding, but it's important to have a good 3D and rendering potential.
Consider the advantages of upgrading a used workstation that was designed for CAD as they are made to run reliably for very long periods, are quieter, and usually accommodate two fairly powerful GPU's. How about:
Dell Precision T3500 Xeon @ 2.53GHz 4GB RAM 500GB HDD 61468AL > Sold for $49.95 + $30, shipping
That has a 525W power supply and supports two 150W GPU's up to 10" long. Keep in mind that a SFF case will be very limited in the GPU size.
The T3500 also supports up to 6-core CPU's such as the W3690 6C@ 3,47/3.73GHz - about $100. There are many good 4-cores and the Xeon X5687 is 3.6 /3.86GHz. I have a T3500 with a Xeon X5677 4-core @ 3.47 /3.73GHz -$35 now.
The T3500 uses DDR3-1333 ECC and is PCIe 2, SATAII, and USB.2.0, but you can add a 6GB/s RAID controller- PERC H310 and a USB 3.0 PCIe card.
If your budget is a bit bigger, you might do something along the lines of my current WS project: my 3rd Hewlett Packard z420. These use up to 10-core Xeon E5's, DDR3-1866, and are PCIe 3, SATAIII, and USB 3. The standard power supply is 600W. I bought one last week for $137 with a Xeon E5-1603 and 4GB of DDR3-1866. That's non-hyperthreading and non-turbo 4-core @ 2.8GHz- the slowest Xeon E5 ever made! However, for $80-90, I can buy a Xeon E5-1620 V2 which is a hyperthreading 4-core @ 3.7/ 3.9GHz. I've already bought and installed a GTX 660 Ti - $55 and the Passmark 3D score went from 60 to 4,783. On Thursday, I'll receive a Xeon E5-1607 v2 and that is a non-hyperthreading 4-core @ 3.0GHz. That sounds like another very slow Xeon- and it is, but the memory bandwidth is 56.1GB/s to the E5-1603's 34.2 and it supports 1866 speed to the 1603's 1600. The E5-1603 hasa Passmark single thread score of 1416 whereas the E5-1607 v2 scores 1611. I don't think there is
any CPU that would fit the Optiplex with a single -thread as high as 1416. I'm also getting 3X 4GB DDR3-1866 to use with the original 4GB so I'll have a 16GB system. the 500GB drive included in that system is a lower end SATAIII Western Digital Blue, but I have a a couple of Seagate Constellation ES.3's around to use.
Sorry for all the detail, but my idea is to demonstrate that with some research and vigilant shopping you can make a system with a lot of potential for not a lot of money. Start with a system with the basic target CPU and build on it as you can afford it and need the performance. My idea with z420_3 is that for now it'll be my television engine and later- seven /nine months?, I'll buy a Xeon E5-2680 v2- a 10- Core @ 2.8 / 3.6GHz (new $1,723, now about $220) and this system can run CPU and/or GPU renderings when my main system (HP z620 / E5-1680 V2 / 64GB /Quadro P2000 ) is otherwise occupied. I'm in no hurry however.
1. Are you doing any 3D ?
2. Rendering ?
2. What is your budget?
BambiBoom