http://forum.mikrotik.com/t/complete-pppoe-router-failure-looking-for-ideas/18449/1 may give you some answers to the capabilities of an x86 system
Have not tested the 1000 yet, but a single core 3gig system can handle 2700 PPPoE sessions+.. Multi-core will change that as well. See just under a 50% gain when dual-cores are used.