Should the PowerBook G4 have an up-side to possible chip RAM upgrade,
that could be done for a modest gain in performance; page-outs to HDD
are an issue due to the hard drive being used as Virtual Memory. Real RAM
is a bit faster. ~ IF the hard drive has sufficient free-space (unused capacity)
that gets used for temporary VM read-writes and relies on rotational rate of
spin. This can be resolved if one upgrades to an ATA/PATA SSD from Other
World Computing; they offer Legacy Mercury SSD for old model portables.
The other thing, is locate the retail full install OS X Leopard 10.5.x and don't
pay too much. ~ There also is an 'upgrade DVD' official disc that had been
included as promo by Apple with a new system back when 10.5 was Very new.
Those can be used as full installer DVD and will also boot the Mac from disc.
I have one of these; and two full retail DVDs for Leopard 10.5.x; and Tiger, too.
The browser... A good browser with some quirks can be had from TenFourFox.
It is up-to-date such as a patchwork quilt -- with mendings to fix weak spots.
See there are different versions of TenFourFox depending on PPC G3/G4/G5 CPU.
http://www.floodgap.com/software/tenfourfox/
My G4 Mac Mini (Late 2005 1.5GHz) and G4 iBook (Mid 2005) 1.33GHz 12-inch
both use TenFourFox browser. Mini runs 10.5.8 & iBook runs an original 10.4.11.
To find a source of reliable main battery may be an issue. Also some of the G4
PowerBook models use a clock battery; this is harder to locate, but essential.
My first edition MacBook1.1 (13-in 1.83GHz coreduo) runs Leopard 10.5.8 too;
and runs circles around the older PowerPC chipped G4; an early Intel-model
but the upgrade path stopped at Snow Leopard 10.6.8. I've no install DVD..
..Note there aren't any custom browsers for early Intel-based Macs..
Good luck & happy computing! 🙂