Sure, here you go. This is an Intel processor option.
Motherboard: The motherboard is the primary component of your computer. it's also the most pain in the *** component to remove and replace. This is the board I'm currently looking at upgradeing to myself.
Asus P5ND2-SLI It supports SLI, which stands for Scalable link interface, allowing you to link two identical Nvidia graphics cards together and act as one card. You don't need to add both at the same time, but with your Autocad requirements, it wouldn't hurt.
http://www.newegg.com/Product/Product.asp?Item=N82E16813131578
$95.00 It also has onboard surround sound audio. though you can upgrade to a soundblaster X-fi later on too if you like.
Processor:
Since you identified Autocad as an interest, and Intel works quite well for autocad, I'd recomend going with an intel Dual Core processor. There are a bunch of different options here, and two main flavorss, the standard and the Dual Core Extreme edition. The dual core extreme edition has 2 processor cores with hyperthreading, giving you expectivly 4 processor cores. The standard processors do not have hyperthreading. Expect to spend between $350 - $1000 depoending on which one you chose:
http://www.newegg.com/Product/ProductList.asp?Brand=1157&N=2010340343+50001157&Submit=ENE&description=dual+core&Subcategory=343&Manufactory=1157&Ntk=all
Ram: 4 512mb DDR2 sticks of Ram. I like Corsair personaly, but they can be a bit on the expensive side.
http://www.newegg.com/Product/Product.asp?Item=N82E16820145526
2x $88.00 = $172
Video card: Nvidia Geforce FX 7800 GTX
http://www.newegg.com/Product/Product.asp?Item=N82E16814122229
$449.00 for one, or you can go with two of the GT models for around the same price, and get slightly better performance overall due to the SLI component.
Hard drives: Excellent time to build a new computer. For they have just released these babies:
http://www.westerndigital.com/en/products/Products.asp?DriveID=190
You can get one of those and use it as a primary drive, or get two, run them in Raid 0 mode, and get close to an effective 18,000 RPM drive. Then i'd throw in a third drive of some sort for just general data. $250 per drive.
Power supply: A 500+ watt power supply, Antec or Enermax are good choices. I run the Enermax Noisetaker power supplies in my cases. $100-$200 depending on model.
Case: Get a full tower, plenty of room for fans. One of the Chenming 602 Derivitives is a good place to start. Between $80 and $200 depending on style and options.
Dvd drives, floppy drives, etc. All that is pretty standard these days, you should be able to get whatever suits you locally. Probably another $100 for those and cables, etc.
That's what I'd build at the moment anyways.