There is a lot more too just sticking a 4 port NIC card in a PC to get actual throughputs. I have lots of customers call asking why their 4 port NIC in this nice Quad Core system don't give more then 40-50 meg throughput. There is buss limitations, there are irqs and interrupts etc. Making a system that can even move close to a GigE takes more than 300 bucks worth of PC hardware.
As far as PPPoE Users, some of our small boxes we have seen upwards of 300 meg with 2600 PPPoE sessions with properly tuned systems. That is on our low end unit with only a dual core P4 in it! We also have seen 5k users on some of our larger boxes, though these systems were moving a bit over 200 meg on their 20 GigE ports.
I would suggest that you use supported and tested hardware. If the RouterBoard can not handle what you are doing, hit up the Made for MikroTik program. I'm sure you can find something that you can pull out of the box, and power on! Thats what we do.
, MCTCE, MTCNA, MCTCTE, MTCWE, MTCNIE, A+, N+, MCP, MTCSE Mikrotik Certified Consultant / Trainer
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-- Author of "Learn RouterOS: Second Edition"