Crystal can't do anything that BusinessObjects Enterprise (BOE) cannot. However, BO can do things that Crystal Reports can't, but Crystal Server can. The major difference is the number of users who can access BOE is either based on the number of named user licenses purchased or unlimited if you purchase the license by CPU (which is
very expensive). There is no "Concurrent User" license available outside of the CPU license - you have to purchase a license for each user. If you're only going to be using Crystal Reports in BOE, you can purchase "Professional" licenses to get access to Crystal only instead of "Premium" licenses that provide access to Crystal, Web Intelligence, and Desktop Intelligence.
Business Objects does have special versions and pricing available for BOE for small to mid-sized companies so that it's not as expensive as the full product suite. You'll need to contact them or one of their partners to get pricing.
If you only need to display reports on the web (as opposed to schedule reports to run at specific times and then provide the "instances" to your users) you can write an application using one of the Crystal SDK's (Java, Com, or .NET) and bypass the whole Crystal Server vs. BOE question entirely. You could also write an application to schedule and run reports and save the instances to be viewed later, but that can be very complex.
-Dell