I take it you've considered USB hubs but don't want to use one?
If I had a Mac Pro, I'd certainly prefer an internal solution. A hub takes up not only space but uses another adaptor. My Belkin hub (conveniently) has the power brick on the plug too, which takes up two spaces on my power strip.
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http://www.compatible.nl/distribution/pex400usb2-4-port-usb-20-pci-express-card-p-2283.html this site says it works for "Mac G5 and up" But who knows...
I use Sonnet Allegro Express USB 2.0 cards in my Mac Pro and can recommend them. The Sonnet has 4 external ports, 1 internal.
http://www.sonnettech.com/product/allegro_express_usb.html said:Technical Notes:
(1) Deep Sleep not supported under Mac OS X Version 10.3 or 10.4.