|As Maximilien says, one server per role. I would put all financial activities (accounting, billing, etc.) on the same server
Any decent financial package should allow copying the data from one machine to another - via backup and restore to the new machine, if nothing else. I would make sure that the seller has a documented method for transferring the software licence. Make sure of this before purchase!
Where financial data is concerned, backups are extremely important. Nothing is more important to the Government than money.
The specifications for the server depend very much on the requirements of the accounting etc. software. I would endure that the server is powerful enough to run all software simultaneously. You don't want to have to juggle between packages, especially if more than one person (e.g. you, your bookkeeper, and your accountant) are all using different programs simultaneously.
Mention of the accountant brings to mind another thing. If he/she is an external contractor, he/she will need access to your financial records over the Internet. Do you have appropriate security for the server?
Lastly, tax packages are typically validated for a given tax year, and are sold as subscriptions. Don't forget to budget for the subscription!
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