I currently run 3 differnet computers with differnet setups.
Computer 1: dual p3 1.2Ghz processors, custom built video card (dual geforce 2 processors with 256 megs of ram), 1 gig of 133 ram. Used mostly for video editing and cad.
Computer 2: Amd 2800+, Radeon 9800pro video card with 256 megs of ram 8x support, asus A7N8X Deluxe motherboard, 3 gigs of 400mhz DDR ram, turtle beach thx audio card. This computer is used mostly for gaming and graphics programs, ie 3d studio max, photoshop, quark, bryce, maya, indesign, ect. This computer has been bench marked and is faster then just about any dual p3 p4 computer on the market. The greatest part was the fact that I got all the I built the computer for under 1300 bucks, minus an monitor and speakers.
Computer 3: Mac G5, strictly my work computer, all graphics computer, no games, This computer is just raw power, and thank God I didnt have to buy it, cause they cost to much.
So really the computer is us to you. If you are really considering a computer for multi tasking and Cad I would recomend a dual processor p3. They usually cost less and have more porcessing power. As for a single AMD vs a single P4, I would take the AMD hands down even is they cost the same. They P4 may have a higher processing speed but the AMD doesnt have the same bottle neck in the design as the intel. Simply put, the amd runs smoother, multi tasks better , has less fault errors, and responds to upgrades better then the p4, is memory...buy as much memory as you can afford.
Just a side note, if you do go with the P4 make sure to buy RDram. The P4 runs much better with this type of memory and runs a higher bench mark with it over the DDr ram. Sure the DDr rams is cheaper, but remember you get what you pay for.
Sorry for the long responce,
DAn