First, don't invest any money in that machine in upgrades. (Unless you can pick up another 1GB or so of RAM for $5). You can get into a used early generation Macbook Pro (with an Intel C2D) and put a SSD in it for $300 - $500.
But just try it (the G4). It's free to evaluate. You should at least be able to track small stuff like 16 tracks. Reaper is made to work with a wide variety of hardware (even Windows machines
).
Hardware cards for that machine that would do anything for you would be the older Protools HD system. Not a good investment here in 2014 unless someone nearly gives it away. Again, a C2D Macbook Pro with Reaper would run circles around it.