I own a 2007 Mac Pro with 2 x 3 GHz Dual-Core Intel Xeon processors.
Probably since moving to Lion OS10.7.3 (now) I have noticed it seems to be slower. Also I am getting more page reloads in Safari (where the page goes white for a second or so then re-loads) This may be that I am trying to do too many things at the same time.
I currently just have 4 x 1GB of memory. Would adding more RAM, eityher an extra 4 or 8GB have a significant effect on the speed? Or has the time come to pay a lot of money for a new machine (either a new Mac Pro if they get updated this spring or an iMac?
It's hard to say without more data, but in general more memory is good.
You should start, though, by checking /Applications/Utilities/Activity Monitor.app
This will give you an idea of the load on your machine, both from a CPU and memory perspective. It should indicate whether you're stressing for CPU cycles (warrants an upgrade) or memory (need more memory).
With respect to the Activity Monitor and memory usage you should read this,
Key values to look at are page ins, outs, and swaps used. If you are doing a lot of paging you may need more memory.
Also when you use Activity Monitor make sure you select "All Processes" for the Show popup menu at the top.
What are kind of "normal" values:
Impossible to tell - everyone is different. Besides, you didn't include the most important element, which is how long your machine has been running since the last reboot.
You see, he page in/out numbers are cumulative. Paging out 1.7GB of data is insignificant if the machine has been up for weeks or months, but if it's been up for a few minutes then it might be cause for concern.
So what's more relevant than the absolute numbers is the rate of change.... page outs rising quickly means your system doesn't have enough memory to run all the current tasks/apps, but it's normal to have some on any system.