I get that often in FW CS5. I have a whole website with many pages in the document. It doesn't occur all the time and even when it does occur it manages to save the document so I just ignore it. Who knows what's causing it - the error dialog isn't very informative. If you right click on the task bar and select "Start Task Manager", then click on Performance tab you will see how much memory is used by the system or in the Processes tab you will see how much memory is used by Fireworks (process name: Fireworks.exe). You can't tell how much memory to allocate to Fireworks, I believe it will use as much as it can. With 8GB (and if you have 64bit version of Windows 7), I don't think you have a memory problem.
Fireworks CS5 (it is a 32bit app) can use up to 2 GB of RAM total, so it doesn't matter how much RAM you have installed.
How well Fw handles memory and errors also depends on how complex the opened file is, how big it is in size -- both in megabytes and in pixels -- and how many Pages/Layers/States it may have...
7251 x 8054 pixels? Wow. This is what causes the issue...
Fireworks is mainly a tool for screen design. It handles pretty well files of approx. 2000-3000 px (width and/or height), but not much larger than that.
In my personal experience, I can say that Fireworks is pretty fast and stable when file is under 2000-3000 px in size, but if once you make the file larger than 4000-5000 px, Fw can become a bit slow, sometimes unstable. After 6000 px, I rarely can work normally -- the biggest file I was ever able to edit and successfully export was ~ 9000x6000 px, but at this point Fw becomes too unstable and slow...
Do you need such large size? Are you preparing something for screen? Maybe you can split the file into 2-3 smaller files?
If anyone's experiencing this problem, a simple fix that worked for myself was simply copying all of the material on the file into a new file - I don't know whether this will work with more complicated files than the one I was creating, as mine contained only vectors, lines, text and one bitmap image.