|CAD like Photoshop does run better with more memory but you have XP and probably 32bit so your installed limit will be 4GB and your actual recognized limit will be a bit over 3GB (probably around 3.2GB).|
Note that what graphics you are running can effect the speed images are rebuilt and displayed.
If you are running a custom built system (not a factory built one, not a laptop), you may be able to overclock your system for additional speed. For advice on this you would need to post a list of all of your components. I am running an E8200 at 3.2GHz currently.
Your memory should be run at a Ratio of 1:1 with your CPU. For the E8200 at stock (FSB1333MHz) which runs at a Frequency of 333MHz, you would also run your memory at a frequency of 333MHz (DDR2 667MHz) which is an underclock to achieve the optimal Ratio. If you were able to run (Overclock) the CPU at a Frequency of 400MHz (FSB1600MHz), you would then run your memory at the same 400MHz (DDR2 800MHz) for your optimal ratio and a better overall bus speed (FSB). A Frequency or 400MHz with a Multiplier or 8 will give you a CPU final speed of 3.2GHz if your system is overclockable (not factory built). If it is a factory built system then you should just underclock your memory to run in step with the CPU's speed.
Eventually a new system, Core-i series, Windows 7 64bit, 8GB or more memory, graphics card with an additional 1GB or memory, Fast hard drive, etc. would be your answer. If you will be running Photoshop also (very likely for a design or architect program) you can read up on (Google) 'Optimizing your system for Photoshop and AutoCad'.
You have to be a little bit crazy to keep you from going insane.