Yes to both questions, although you may need a bit of help with OS X. If you indeed have a Beige G3, go to
www.macsales.com and look for a free app called XpostFacto by Ryan Remple. It allows you to install OS X on an "old world rom" machine. Any later model G3 will run OS X on it's own
Be sure to check your ram, as OS X needs a min of 512mb to run well, plus a video card with at least 32mb of vram, (I recommend a Radeon 7000 or 9200) and around 4GB of free space on you HDD too.....
As for USB, I recommend staying with brand name PCI cards like Belkin or D-Link. They are reasonably priced, reliable and plug & play in OS X
Depending on the CPU speed, you may at some point want to consider a CPU upgrade card too, but thats another topic for later on......