Is it possible to give some idea of amount of users and likely concurrent users?

If you are serving 20 people with simple reports (with generally standard requests that can be cached) occassionally during the day - then it wont take much. If your serving 1000`s of people with hundreds of complex reports (also varied in nature of the parameters sent to the report) then you might want to look at a farm.

Also are you talking about the reporting services front end, back end, or both on one box?

I am sure you have looked at the SQL minimum specs. Well i wouldnt run it on that. Tell you the truth i recon you could struggle to buy a server at that spec now. The recommended spec is fine if your running only a few reports.

One thing i would consider (if possible) is upgradability. So CPU is a bit of a cost and pain to upgrade - so think a little longer term. RAM however is cheap (its been a year or so since i worked on servers - i guess its still cheap 🙂 So buy a good CPU. RAM you could buy it now - but you could also add to it a year down the road with minimal downtime and cost (IIRC).

Hope this helps.

Dan