I have a Mac that is also a 1,25GHz, of course it is not speedy like the iMac 24" I also have around here, but it is still a really good machine which runs 10.5 with no hassle, I don't regret about having updated from 10.4 to 10.5, albeit 10.4 ran wonderfully on it.
See, a dual, quad core Mac is not going to make your Finder going snappier, instead it is the heavy stuff which most beneficts from all those additional cores as long as the given app knows about them… video apps for example would love them. Thing is your PB with 512 MB is really too low on RAM, just open Activity Monitor and check how much RAM is used and how much is free.
Four types of memory appear in the System Memory pie chart: Wired, Active, Inactive, and Free. "Used" is simply the total of the first three. Don't get confused about Inactive and Free RAM.
The total of the four types equals the amount of random-access memory (RAM) in your computer. RAM as you know is used to store information that is in use or used most recently (think about scrolling huge iPhoto libraries). Information in RAM is loaded from your hard disk, and the RAM is emptied when you turn off your computer. Makes sense, huh?.
The "VM size" refers to virtual memory, a system of putting information in RAM or caching it to your hard disk as needed. Thus Mac OS X can "virtually" use more memory than the amount of RAM you have. The hard disk is much much slower than RAM, so the virtual memory system automatically distributes information between free disk space and RAM for efficient performance. "Page ins/outs" refers to the number of times Mac OS X has moved information between RAM and disk space. You don't want to have your hard disk "playing RAM" so to speak.
Wired memory
This information can't be cached to disk, so it must stay in RAM. The amount depends on what applications you are using.
Active memory
This information is currently in RAM and actively being used.
Inactive memory
This information is no longer being used and has been cached to disk, but it will remain in RAM until another application needs the space. Leaving this information in RAM is to your advantage if you come back to it later.
Free memory
This memory is not being used. Free RAM is wasted RAM and Mac OS X knows about it, the more RAM you have the more it will use. Always.
What does all this mean?
This means you shouldn't worry when the Free memory is low. The only time Free memory should be high is right after the computer starts up. As you use applications or services, memory is used and transitions to Inactive. Applications that need more memory will take from the Inactive, but the Inactive is there just in case you need it again. If the combination of Free and Inactive is very low, then you might need more memory.
Make no mistake, an old PowerBook like your with let say 2 GB RAM is going to be speedier than your MacBook with 512 MB RAM cause the later is poorly -too low- loaded of RAM.
Of course, common sense applies, stuff like Office 2008 is going to run slow no matter how much RAM you have in your PowerBook. But that's another history, blame Office 2008.
