Thinking Of Upgrading, to address these needs... Options

Here is what I have: G4 Cube, 450mhz, 876mb RAM, OS 9.2.2, Cubase 5.0, soft synths (Halion and Absynth mostly). I've run this computer into the ground and am thinking of finally moving on for numerous reasons (I do video as well as music).

Because the price on them is nice right now, I'm looking at a dual boot G4 Dual 1ghz, or 1.25, or 1.42 ghz. And hope to still run it in OS 9.2.2 and stick with Cubase. I'll ug the software when my budget allows.

I'm hoping to improve:

1) Latency in soft synths. - As the future nears, I use more and more soft synths, and little else. I can barely tolerate the 50ms latency in Cubase 5.0. How much CPU power can fix this, and how much of a factor is the sound card? (No possibility of adjusting Sound Manager latency in Cubase in the Cube. Nor can I ug the sound card).

2) I can stack a few tracks and sounds up, but eventually overload the CPU in Cubase. How much CPU power do I need to go to really push the thing with soft synths and internal signal processing? And how much of a difference here does a better sound card make?

Will one of the DP upper G4's be that much better than what I have? And what if I put a quality sound card in it, how much does that matter with soft synths and latency anyway? And lastly, what about Firewire or USB external units? I know they are mostly for getting audio in (some Midi too, I know), but if I am already 95% soft synths, and could live with 100% soft synths, would this just be a waste of $?

I am considering moving to G5 in about a year instead of jumping to a better G4 now. But that will require much more $, mostly for software, as I refuse to buy pirated/hacked and went legit long ago. Would sticking with what I have (because it does work, with little or no bugs) and waiting a year for this be a better way to go?

(Don't ask how I get audio into the Cube and Cubase now, it's a real pain in the rear). Mostly I just love the simplicity, and power of soft synths. I know it's not for everyone, but for me, they're great.

Sorry for so many questions. I notice that readers on here seem quite knowledgeable and like it when people are specific with their equipment, and their needs, so I hope this helped. Answer whatever you can.