I will give you my answer.
The system is designed preliminary for CAD driven applications correct?
These applications should be used in a workstation/server environment therefore if I myself had to design the system on a high level budget I would be using a Xeon system. The Xeon's are designed more for working with CAD applications than their equivalent desktop processors as well as large database systems.
However the desktop variant which you have specified, Conroe, does have the ability to perform these tasks at an acceptable level if paired with the correct hardware configuration.
With CAD if you are to design something to high level it requires a substantial amount of processing power and memory storage to complete the rendering (etc) of the image, design or equivalent that is being worked on.
However I do not wish you to take the design I wish to use seriously as it will over budget the system drastically as the CAD design work I am used to requires high end quad Xeon servers to process the information at acceptable rates.
I will therefore give you a Conroe based design to work with.
Please Note: The system that I am going to list will be using a 32 bit operating system and hardware configuration solely because Core 2 Duo has problems with the basic x86-64 instruction set and can not attain an acceptable speed at CAD packages using the specified instruction set.
Processor: Intel Core 2 Duo E6600 (The 4MB L2 cache really helps here as CAD is memory intense, the beauty about this processor is that its cheaper and can be overclocked very well if the additional performance is required).
Motherboard: ASUS P5B (Not much to say here, not super expensive but a very good board especially with onboard latency reduction abilities)
Memory: 4x 1GB memory (this can be basic, CAD does not require huge memory speeds or extreme latencies, it just requires a substantial amount of system memory) so even PC3200 DDR2 memory will do, however I would recommend at least PC5400 especially as then you don't have to run any dividers.
Graphics Card: NVIDIA Quadro FX 5500 (This is very expensive and with a few little modification or maybe none at all will work perfectly in this board, a single one will work fine to save costs).
Operating System: Windows XP Professional Edition will do I suppose
The rest is your decision I’ve just listed the important parts.
Anymore help, either complex or simple just ask, tried to keep this quite simple.