Hello, Gini,
I'm on Mac Powerbook G4 with 1 gig ram (one PB 667 MHz with 16 megas of video, and one at 1 GHz),
and Vue5Inf works fine, here (some architecture).
Well, some problems with the interface that jerks or freezes, mostly when using wacom pen.
I rarely model inside Vue, use architectural VectorWorks for that. -> 3ds -> Vue.
When I began with Vue, as it came out of the box, it often crashed.
I "fine-tuned" the prefs (funnily enough, things began to work as they should, when I pushed up both foreground and background thread quality)
and try not to overload it, not to ask it toooo much for my old puters (no Versailles mirror hall with GI plus volumetrics and glowing mats + lensflare at sundown ;-),
and when working in high-poly-scenes, I "slow down" my usual turbo working speed a little, to leave it some fraction between mouseclicks.
Well, plus the usual dispatching of unused parts to invisible layers.
I get good results, nearly no crash, now.
But they are right for the deadlines, I think :
I make my 'standard' renders with VectorWorks' built-in renderer, where I'm sure of the time needed and of the (basic) outcome,
and use Vue, when I got some time to make trials, errors, and long renders, but want a super result.
Also, compare the puter's performances : the Vue "benchmark" takes ten minutes or so on a MacBookPro, where it takes about two hours on my powerbooks G4.
I dont ask Vue to make superpro work : I haven't got the superpro staff and equipment to do that. I don't regret my money. I don't know an other program, with which I could do, what I do with Vue,  with the same ease (and at same price).
I'm sorry that you don't get it working the way you want :-(