If you have an Intel-based Mac, and are currently using the Leopard OS, then updating your OS to Snow Leopard ($29) could help - or not. Many people are having "issues" with some aspects of the Snow Leopard OS installation and changes, going from either Tiger OR Leopard...
If you have a PowerMac system, running on any version of Tiger, or earlier, then you might have some problems. For one thing, Snow Leopard will NOT install, cannot be installed and will not run on anything pre-Intel. You have a bounty of possibilities, depending on the current version of your OS, be it Tiger or earlier, due to possible version updates you can perform. However, I suspect the OS, in and of itself, is not the specific source of your problems with sluggish operation - assuming that is what you mean.
OK, if you have a pre-Intel Mac, and a dual (say, a Mac G4 dual 1Ghz) system operating on Tiger, you can, after jumping some version update hoops, transition to Leopard. But NOT Snow Leopard.
If you have an Intel-based Mac operating on Tiger, you will want to go through the process to install Leopard (Apple does not recommend installing directly from Tiger to Snow Leopard, and you're not supposed to actually be able to, but I understand from MacWorld and others sources that you can - preventing the absolute necessity of purchasing the boxed version that includes Snow Leopard, iWork and iLife 09). All this is nice to have though.
You might want to reconsider going any further than Leopard, however, if your Mac is not a current iteration Intel 2008-2009 model.
Mostly what I suspect is you need to do a complete system HD backup of your present system. Start with a brand new HD, or wipe your old system drive clean and do a complete re-install of either your current OS, or any of the updates - you MIGHT have to go through a series of version updates to qualify for the next OS installation, depending on where you are with Tiger or Leopard.
Confusing? You bet. But I think the single most effective thing you could do would be to actually remove your current system hard drive, replace it with a 320 or 500 Gig HD, install a completely fresh OS (either your current installation disk version, or Leopard, or Snow Leopard if you have an Intel-based Mac) and run that sucker. You will or should see a significant increase in response. Then you can slowly return some of the other programs/docs from your older HD to the new OS HD.
The second most important thing you could/should do is install however much RAM memory sticks your model can utilize, and that you can afford. If you are attempting to do ANYTHING beyond e-mails and word docs or multi-tasking with less than 2 Gigs, you are overwhelming your system - GUARANTEED!
Others might have more to offer, but I really believe one or more of the above might help you. Unfortunately, costs are, or can be, significant depending on how far you take this. But if you are on an Intel-based machine and using Leopard, and you think $29 for a Snow Leopard upgrade "costs a little" then you might have some problems making things any better with your current system.
