I think it's not too likely that a few duplicate files would be affecting the performance of your iMac.
"Really slow" can be caused by several possibilities:
1. A hard drive that's nearly full - check that you have plenty of free space, at least 10% free of the total available The smallest hard drive in your iMac was 160 GB, so on that one, more than 16 GB free space is reasonable.
2. Using software that is using more memory than you have installed, causing lots of page outs to the hard drive. That will slow your system down dramatically. When you get a slowdown, open your Activity Monitor (in your Applications/Utilities folder), and make sure the drop down menu is set to show your "All Processes", and not just "My Processes". You can sort the list of processes, by clicking on the header for % CPU, for example, to show the highest CPU use, by process. Click in the bottom window for the CPU window, which will show graphically, how much total CPU activity you have at the moment. Click on the System Memory tab, and notice the amount listed by Free, and Page Outs.
3. If neither 1, nor 2 show you anything way out of normal, then you might be at the OTHER common problem with a 5 or 6 year-old Mac - a possible failing hard drive.
1. you can help by clearing out old files, or apps that you don't use.
2. Lots of activity that are related to high Page Outs might be improved by upgrading your RAM. 1GB is not very much, especially on an Intel Mac. Your iMac Core 2 Duo can be upgraded to 3 GB maximum RAM. If you upgrade your OS X system, you would definitely want to do that memory upgrade.
3. As your system gets older, your hard drive is more likely to fail. It's not too simple to replace, but is do-able.
One test that you CAN do, is to boot to your OS X installer DVD, and run Disk Utility from the menus. Don't waste your time running Repair Disk Permissions. Your goal is to see that Repair Disk passes with no reported problems.
4. If you have plenty of free space on the hard drive, and you can upgrade your RAM to a larger amount, Then I suggest that you bypass 10.5 completely, and upgrade to 10.6
Several reasons for that:
10.5 is still high priced, and not always easy to find now. You can purchase 10.6 directly from Apple, for only $20
http://store.apple.com/us/product/MC573/mac-os-x-106-snow-leopard
Snow Leopard would be a fine upgrade for you, as it's quite stable, and your iMac will run it very nicely. You COULD then go to Lion, but you won't have the advantage that newer Macs have with more RAM installed - and too many folks here will tell you that Lion is a much bigger leap, than going from your Tiger system to Snow Leopard.
Just my opinion...