I'm pretty sure you mean OS 10.5.8 since that's the latest OS supported by your G5.
The only thing you can do is add an SSD. Since your Mac only has SATA1 there is no point in getting a SATA3 SSD over a SATA2 one. The maximum theoretical throughput you can achieve is 187MB/s, which just about any SSD achieves, no matter which generation.
Bottom line: Get the most affordable SSD (SATA2 or SATA3) that suits your needs size-wise. It will be at least twice as fast as your current HDD.