Hi Margaret, welcome to the forum.
The (ideal) hardware requirements for a server based Recorder 6 installation are a bit of an unknown at the moment seeing as it's not really being used extensively in LRCs at the moment. I'll be able to tell you more when we begin testing on our server, which will be soon. I'd suggest either talking directly to Dorset or JNCC. Hampshire have also been using a version of 6 for a while now, so they may be able to help.
One important aspect you'll need to consider is how many users will be accessing and using the database at the same time. It could be that if it's 5 users or less and your database is under 2GB in size, then you'll be able to stick with the free version of SQL Server (called MSDE) that comes with Recorder 6.
There are also other factors to consider when trying to work out a total cost of ownership for a SQL Server beyond buying the hardware and software, such as support costs. Who is going to maintain and administer the server and the software?
In terms of licensing, again there's no straightforward answer. You can buy SQL Server in a plethora of flavours, and in several ways, depending on the way it's being used and by how many people. Microsoft's SQL Server "how to buy" page gives you some information (prices are in dollars, though). The SQL Server 2000 Product Overview should give you some insight into the vast array of options available for a SQL Server.
So as you can see, it's not terribly straightforward, and really you need to sit down with an SQL Server expert, or an R6 reseller, and discuss your needs so as to best formulate a solution. Here in Sussex I have the benefit of having the Wildlife Trust's resident IT person who has administered the Trust's server for some time. I'll be sure to share my experiences and let you know how we get on when we begin testing. Any futher information you are able to gather and share would be most welcome, so please do post your findings on the forum.