Gday!
Was just reading up a few benchmark results, found here.

Although I couldn't find a vendor for the chipsets that matched the ones you listed, I did find some result for the chips in cards from different manufacturers.

the radeon 9550 scored from 182-393
the 7600GS scored from 842-1200
the 540 quadro scored 205

There is some variability in speed, depending on which manufacturer has put together a card with the chipset on it, but it's often not a huge amount, and the above figures give a rough idea of the expected performance.

About the 7600 and the 540 I can tell you nothing much else.
The 9550 is a card I've just bought and _for the money_ I'm quite happy with it. There has was the occasional hiccup, but overall, it's been fine. It won't render subds in hardware, and from time to time 'mystery' pixels pop-up in the viewports (like an LCD with dead pixels), but the renders all come out fine.

That said however, without knowing any more I'd be inclined to choose the 7600 gs if it's inside your budget.
UNLESS you were planning on upgrading to a PCI-e motherboard in the near future (assuming the 9550 is a fair margin cheaper than the 7600, AND that you weren't going to sell it afterwards to recoup some of your outlay)

You may have an issue with the way image planes are displayed with a 7600 (they get stretched in a circular fashion, a bit like water going down a plughole) but the fix for that issue has been posted plenty around the forums.

As for the rest of your rig, I'm only using a celeron 2.66 and 1GB and that's fine for my needs. I will point out however that my ram is all but consumed when working with a basic scene consisting of around half a million polygons... Not that I've ever need that many yet for anything other than benchmark comparisons between my current and previous cards.

S.