Some aren't too expensive (for ssd of course) if you can find them stock somewhere.
The real question I guess is why put such a fast, expensive and low capacity in a computer like a mini G4? I'm not saying it isn't a good idea, I just can't find a situation where the disk woouldn't be bottlenecked by something else.
And in that case, you're right amdfangirl, might as well upgrade the whole thing.
@DamianVonBarone: So... why? I'm curious.