That is a horrendous bottleneck for an SSD. :/
No argument there, but if you're using old PowerBooks, there's really no way around that ceiling sadly.
That said, unless you're moving big files, most of the "snappy feel" of the machine comes from the ability to make quick, speedy, small reads and writes with near-instant access times rather than large sequential reads and writes. That's where an SSD can shine even with the old, slow bus.
I totally agree with Frost7...
As a matter of fact, I strongly suggest that the OP gets himself a decent (forget KingSpec) SSD unless (s)he needs umpteen gigabytes. The *real* real-life difference comes in eliminating seek-times, not in max throughput (BTW, show me a 2,5" PATA HDD which systematically saturates that ATA66 bandwidth).
I've had good experiences with a Transcend SSD in a 12" PB (10.4. Boots in 12 seconds flat - you can almost forget sleep), and a noiseless PB is bliss.

Sadly, switching a HDD for an SSD also exacerbates the fan noise - a near constant companion with the 12" PB
RGDS,
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I tried this on my G4 PB 12" and boot times were approx 2-3 sec. faster if that's any help to you.
I'm no OS technician, so I do not know what files are needed for loading the OS, but my experiences point in another direction. Boot times are generally out of another world with an SSD, even with machines having constrictive buses, and this experience comes based on having supplanted boot drive HDD's with SSD's in a total five computers (MacBook (Alu), MBP 15" (early 2011), MBP 13" (Mid 2010), PBG4 (12") and MacPro (3,1)) (all SSD installations based on cloning, so no difference there either)
EDIT: I knew I had my records somewhere, so I went searching: 12" PBG4 (1,5 Ghz, 10.4.11): Boot time with 60 GB 7200 rpm HDD: 38 secs; Transcend 64 GB SSD, 12 secs flat. So, my experience is more like 3 times faster than 3 secs faster...
RGDS,