We've been testing BES 4.0 for Lotus Notes, with 6 users, and had all componets on a high-end PC. We're now procuring a production server and will have about 150+ users. We're going to run the Router separately in the DMZ. I can find specs for the BES but what would be typical specs for the router. I have to give up the high-end PC but do have lower-end PCs available for possible use for the router

I haven't seen any typical specs for the Router before...however if you were to breakdown what it would actually be doing....then you could arrive at what the specs should be.
Since this "piece" of the BES environment would be involved in communicating with the wireless network and also routes data to handhelds that are connected using the BlackBerry Handheld Manager (least cost routing option)....it would require a fair amout processing speed and memory utilization. It would be safe to say that a high-end PC would be sufficient, as a minimum, to handle the load. However if this router is placed in the DMZ and if there may be additional BES servers connecting to it I would recommend a dual P4 with ~1GB RAM. Similar to wht you might expect from a SMTP or gateway server in a typical messaging environment.
Hope this helps.

I put our Router on a P4 3.0 with 1GB Ram and a simple Raid 5 array. It sits in our DMZ and it is natted back to the main Domino/BES server. It really is just a relay so it does not have to be very powerful.

I agree with ocman. I doubt you'd need anything too terribly robust in order to run this. Granted, it is a vital part of the environment if you want to utilize it separately, but considering it does the same thing on the BES (sans network traffic), I don't really see it as being equally as robust as the BES if it is separated. I believe the memory horses for the BES are the Attachment service and MDS (and SQL if run locally).