Just max out a 24-inch iMac and save up for a Lambourghini.
For an average consumer, the iMac is more than enough... two 30-inch displays would take a ridiculous amount of desk space, and you would never really use them both.
There is no point in buying something just because you can, if you aren't going to get anything out of it.
Lambourghini > Apple.
I agree. If you don't have any reason why you need that much power (and we're talking about a STUPID amount of power), I don't see why you would want to spend $24,000 on a computer, just because you can.
Like a lot of other posters have said, I would buy a rid-range Mac Pro and a 30 inch Cinema display, and put the remaining $18,000 in a high interest account and then you would never have to worry about upgrading again!!! The interest that ticks over will keep that account topped up so that every 2 years you can upgrade. Remember, a $24,000 system still becomes obsolete. In 6 years time, an iMac will probably have the same, if not more power.
Plus, in Windows (32BIT) the O/S can only handle a max of 4GB ram anyway. If you have the expensive GFX card with 1.5GB RAM on it, that leave a max of 2.5GB of adressable RAM. Only a 64 Bit O/S (Leopard/Vista Ultimate 64 Bit) would be able to take advantage of any additional RAM.
I would be VERY surprised if you noticed the difference between a mid-range Mac Pro and the overkill system for the tasks that you are doing (even the CAD programs).
This is an example of a still VERY capable and powerful system:
# Two 2.8GHz Quad-Core Intel Xeon
# 8GB (4x2GB)
# 300GB 15,000-rpm SAS
# 1TB 7200-rpm Serial ATA 3Gb/s
# NVIDIA GeForce 8800 GT 512MB
# One 16x SuperDrive
# Apple Cinema HD Display (30" flat panel)
# Apple Mighty Mouse
# Apple Keyboard + User's Guide
# AirPort Extreme Card (Wi-Fi)
Cost: $7,248 USD (saving ~$16,000)