Amoda,
I don't know your technical level of understanding so let me break it down for you, if you have more questions feel free to ask. When a programmer writes a program, they are 'compiled' very specifically, not just for operating system, but also for what's called the 'Processor Architecture', you may often hear it referred to as an instruction set, or something similar.
The most common types are ARM (Mobile CPU's, technically the most common, found in your iPod, iPhone, etc. Not used in desktop or laptop computing, but that may change!), x86 (There are lots of sub categories here, like i586, AMD64, etc., but x86 is what an intel Mac is, it's 64-bit, which means it can run both x86-32 {i586} and x86-64 {AMD64} code that has been compiled for Mac OS X), and PowerPC, which is based on the RISC architecture.
What this means for you is, basically, unless it is SPECIFICALLY noted, Intel Mac software will not work on a PowerPC Mac. OS X 10.6 and up aren't 'locked out' of PPC Macs, they literally cannot run because they are only compiled to run on an intel mac. You may hear people talk about 'Universal Binaries', those are programs that have been compiled both for Intel and PowerPC macs. You won't find those in new software most of the time, but older software.
In short, you can't run newer software. Some stuff is occasionally supported on PowerPC, but for the most part, your restricted to older software. Now, you can still browse the web, run software that is only a couple of years old, and stuff like that, but you cannot run Intel Mac software. Technically you can, through emulation, but that powerbook lacks the processing power to do so.
Other thoughts, if I were you I'd run OS X 10.4, not 10.5. The reason being is that 10.4 allows you to run OS 9.1 software, so it opens up your software library even more.
If you have any more questions, feel free to ask! I find this is a pretty friendly part of the forum.
As far as your question about an SSD, SSD's are not made on the IDE (PATA) interface (there are plenty of platter-based hard disks that can max out that bus speed). So in short, it won't work in your PowerBook. To be honest, for the money you would spend on an SSD, you could get an inexpensive Windows laptop that would outperform that machine. Windows isn't exactly a 'good' computing experience, not like OSX is anyway, but if your just looking for an ultra portable web browser, light use, it could do the trick. I like Acer, myself, but stay CLEAR AWAY from HP. There is a reason they consistently rank low in customer satisfaction and reliability.
-John