OK, then you won't be able to use all 512 kbps for audio since there will be packet overhead.
So you'll probably end up using 384 kbps for the codec bit rate. If the codec permits adjusting the packet size you will have some control of the delay vs bandwidth tradeoff.
If you want to run *anything* else over the link you need a managed switch and proper configuration.
As soon as you can put two PC's on the link and run ping -t constant pings and look to see what the results over time.
Even better download the colasoft ping tool (free) and use that to test the link. These results will help you set an appropriate buffer setting (use 1.5 times the longest round trip ping and you should be robust.