Hi Gary,
Well. Volumetrics always take time.
Render tips

or how to make your PC run slower for no appearent reason)
1. Volumetrics. This is basically a texture that seems to add depth. It makes nice fires and smoke. It is occasionally used for other things too. Avoid it whenever you can.
2. Transparencies. These seem to not impact quite as much as volumetric materials. But, they still add a good deal to the render. Avoid these when you can. Use them in place of volumetrics when you can. Fuzzy transparencies have the same bug (over lap them and they destroy each other) as volumetrics. And they seem to seem to really hammer render times too.
3. Fuzzy materials. Unless it is a fuzzy transparency it is not as bad as the first two. But, it is still quite a hit.
4. Reflections. They mess up most renders at Civ scale, and can add a little or a LOT to your render time. How much these add in part depends on the complexity of your object. For an example and for fun, try a highly reflective sphere with just a photograph applied to a box to fake a background image. Next do the same with two Torii (doughnut shaped primitives). have them interlock, but not touch (like a chain). There you have reflections of reflections. Ugh.
5. advanced render options. Bump up the rays a bit too much and go on a camping trip before it is done. AA above normal is pretty much a waste for Civ. TIR, rays per pixel, etc... default is fine.
6. Texture size. Big honking textures can take a LOT of time to render. It can have as much or more of an impact to render times than poly count.
7. Poly count. This is the number of sides an object has (when you view the wire frames). The actualy number of objects has a part too. Bryce made (ie; not made in a modeler) always render much slower. The big reason is polygon count. They never discard unused polys the way you can in a real modeler.
8. Running Bryce too long. Bryce has a memory leak. If you run it for too long it slows down. Wyrmshadow and I have a bunch of ram, so we don't suffer too bad from this. but, with only 512mb... it is something you need to keep in mind. If you have somewhat recently started bryce and have not opened and closed a bunch of scenes, you're ok. If you have opened and closed scenes or just used it for a while.. Exit Bryce and restart it. This clears up most of the resources Bryce uses. If you really want maximum performance, restart the PC.
9. The color yellow. I do not know how true this one is, but I heard from a sincere and knowledgeable person that Bryce has (since version 3) had a minor slow down when yellow is used.
2.8ghz, is fast enough for bryce (it does not matter if it is an AMD or Intel). But, 512 is a bit of a problem. I really reccommend 2gb, if you have the resources to get this done. 2gb is the perfect amount for Bryce. It can use ALL of it. Above 2gb, and Bryce does not make full use of it. But, if the OS and other stuff has a little bit for itself, this can leave more of the magical 2gb amount for Bryce.
It is possible to make a unit with out AA. You can also experiment with rendering the frames slightly over sized and then resizing them down in Photoshop and letting the paint program apply AA for you instead of applying them at render time. I found that when I work on my laptop, this is faster.