You will not be able to use a UAD-2 card, unless your G5 is one with PCIe slots. Most G5's had PCI, or PCI-X, I believe.
However, the plug-ins available on the UAD card are among the best out there, imo.
Duende's plug-ins are excellent as well. I used to own Duende, but it didn't play nice with my UAD-1 card back when I had a G5. I wasn't willing to lose my UAD plug-ins. so Duende was removed from my set up.
Waves SSL-4000 plug-ins sound good, but aren't as full or rich sounding as Duendes. Still, I use mine a lot.
A fully native system with a Mac Pro is tempting. DSP solutions like UAD and Duende certainly sound great, but there can be issues. When the UAD-2 card was first released a few months back, it had some serious issues... I know, I was among the first reporting these to UAD, but they have worked out most of the kinks now.
I never did use Duende when it wasn't causing havoc on my system (when used with UAD-1 plug-ins), but I've read those issues did eventually get sorted out.
Tough call... I think either way, you will have a great sounding set-up. The advantage of going with a new computer keeps you current, being able to take advantage of the newer OS, as well as future Logic updates optimized for the new OS (if we ever see them).
But if your G5 is PCI slots only, the new UAD-2 card isn't an option for you.