Even for a Mac, four years is ancient. I wouldn't be surprised if you opened up your machine and found it full of white beard hair.
I have just abandoned MY four year old G5 dual Mac - but with a 256 Mg video card [to the wife]. 64 Mgis hardly enough to generate an OpenGL window fully rendered. 128 or 256 is swell.
You have a minimum system there for rendering - You should have a minimum of 2.5 Gb RAM to run that machine AND Archicad rendering at the same time.
Here's what you do:
Recommended: Drop $4G on a new machine with enough RAM [16GB] and a 512 Mg Video card. Zoom.
OR - for no money, reduce the size and quality of LightWorks renderings.
My book has a number of suggestions and identifies pitfalls - like overlapping refractive glazing - that unreasonably extend rendering time.
OR: also for no money, abandon photorendering. Stick with OpenGL textured views. Most viewers accept a coarser transitional animation that cross-fades to a fine still rendering. This can happen in near real-time.
OR, for less money than a new machine, Get Artlantis. It makes images faster and prettier than LightWorks, has a smoother camera control, and has many other productivity benefits like previews. RECOMMENDED.
Clarification: While the CPU BUILDS the 3D view, OpenGL renders it as you navigate in 3D. OpenGL primarily uses the RAM on the video card to do this.
Dwight Atkinson