well afaik, the only game in town i've seen for proc upgrades is newer tech -- newertech.com/products/processor.php -- and for $400 you could conceivably get a radical speed boost. you'd be bottlenecked by the bus speed of your ram, of course. but it is possible.

but.

for $400 you are 1/3 of your way to a new iMac, which would kick the ass up and down, back and forth of that machine even after the upgrade. Honestly, that machine is at least EIGHT years old. In my opinion, you are beating a dead horse.

Will it speed up the processor? Definitely. And it will make a large difference in some thing.

But processor speed isn't everything. For some applications you could put in a processor 10,000 times faster and it would have little effect.

For some things to speed up you would need a faster hard drive. So I would suggest increasing the speed of your hard drive, too.

For other things, you need a faster bus. Unfortunately, you can't change that, but you can max out our RAM. That should help a bit.

For other things the speed and power of your graphics card is important.

So, if you really want to speed up your Mac--for all general purposes--you install a new hard drive, a new graphics card, a new processor and more RAM. The cost--assuming you are easily capable of doing the work yourself, will be about $800-$1,000 dollars. Do that any you'll have a computer that doesn't have numerous modern features and won't run some software.

Oh, and if your monitor is over three or four years old, it might need to be upgraded, too. The colors, brightness and contrast might not be up to spec.

Now, the question is, will doing only part of a complete upgrade give you the boost you want? I don't know. It depends upon what applications you regularly use.

If, for example, you primarily surf the web, you can probably get buy without upgrading your hard drive. If you do a lot of converting of files from one format to another, or rendering video, you might get along okay without the new graphics card. If you had been upgrading so that you already had maxed out RAM and a faster hard drive, then getting a processor and video upgrade might let you continue using your current applications for another couple of years.

However, if you have to make all of those changes, a used or refurbed Mac might be a better choice. For example, from the Apple Store you can currently get a refub iMac (from the previous generation--last August), starting at $950. That includes a 20" monitor, faster bus, faster graphics, and a 2.0 GHz processor. It will run all of the latest software. That's about the same price as upgrading everything in your current computer without a new monitor and without an Intel processor, a faster bus, etc.

In my opinion, spending $400 to upgrade the CPU on such an old machine would be a waste of $$. It really depends on what you are doing on it, and how much of a "power user" you are. For me, working on a G4 is torture compared to my dual G5, and even that seems slow compared to the new (very expensive) 8 core models. But I work on my machine full-time, have lots of RAM and CPU hungry applications running at a time, etc.

How much RAM do you have? Too little RAM can be a real performance killer.