not to be picky, but you say you 'need' a g5...
why is that? i'm not saying there's anything wrong with a g5, but it was replaced by the mac pro in 2006, and is probably beaten in performance by many modern lower spec C2D options...
plus, the g5 is the last line of ppc processor macs, which are not supported by snow leopard and future updates, so really you're buying into a dead end..
also,,you say you need a vocal booth,,,,,,now, i don't have much experience with booths/treatments, but every indication on the forums seems to say that a well treated room is a better choice than a vocal booth.
do you 'need' a control 24?
i use one every other day at college, and at home i use a 003, and tbh i don't feel that the 003 holds me back at all.
if you need more than 18 inputs, that's fair enough, but if you could get by with say 12 (003 gives maximum 18), i'd recommend it instead, with an 8 channel adat unit like
the focusrite octopre le.
i'm guessin that you mightn't be needing a huge number of inputs cos you mention 'a good quality microphone' rather than loads of em, but i could be reading too far into that.
i know that going 003 will mean you're on LE rather than HD, but again i don't see what the fuss is about and you're gona save a few thousand goin that way.
The only advantage that i actually see in using the two systems is that HD does automatic delay compensation but,,again, it doesn't bother me too much having to do it manually...and there are plugins that you can buy to do it anyway (
mellowmuse ata)
i don't see any advantage to the processing power of HD systems,,,not with modern computers being as powerful as they are....i've mixed pretty heavy sessions on a single quad core 3.4ghz w/ 8gb ddr1066, and never experienced a hitch, even with low hw buffer settings.
a 003 LE setup will leave you every bit as compatible as an HD rig, but if you do decide to pursue HD, you will need to buy (and feel free to correct me here) at least one hd core card, and an HD interface (192?)
i've often heard the control 24 referred to as a 'big keyboard' or a 'big mouse', and tbh it's totally true...there's a lot more to an HD rig.
hope that's useful.