first of all - if that Dual Mac is a so-called 'mirror door' (MDD), forget about it immediately - it will drive you nuts.
Either by obscure errors or by an amok running fan.
A Quicksilver or even Sawtooth (probably a bit slow for a sequencer, even in Mac scale) are reliable machines - if you want to go OS9.
Is there a special reason to go from XP to OS9 ?
A dedicated PC box with the Intel 865Perl mobo (best price /performance/stability ratio) with a (say) P4/2.6 isn't more expensive.
The famous 'Mac stability' is a myth regarding late OS9 machines, in particular with late revisions of that OS.
CWA has never been very good in Mac programming, politely expressed
Don't get me wrong - I'm not biased as I make a significant share of my living with Mac consulting and support - since 1985 to be precise.
A Mac is easier to install and to maintain and safe from typical internet security crap, but since you just want Scope and a sequencer that's probably no concern anyway.
I run SFP even under Win98 on a 2nd hand P3 (120 Euro) without such isssues, but I wouldn't suggest it in your case, as the sequencer will (most likely) not be that happy with just 512MB Ram
cheers, Tom