PowerPC G5 - overall updates and cleanups

My parents have my old PowerPC G5 and recently told me it's running slow. They've also noticed that their web browsers haven't been working up to par or where they need them too. So I have a couple of questions:

1) Is it possible to replace the processor for a newer one? I've done some research on how to do it, but don't know of a good processor to switch too.

2) Is there a good overall clean program for this CPU?

3) Any web browsers out there that accepts 10.5.8?

Here are the specs:

Version 10.5.8

Processor: Dual 1.8 GHz PowerPC G5

Memory: 2.5 GB DDR SDRAM

Thanks to anyone who can help!!