Payments and user balances/virtual coins I would like to implement some premium options within my website.  But most modules I have looked at, focus on providing payment-interactions (buying items, adding node etc.).  

In contrast, I am looking for keeping a seperate account per user and their purchases.  My expected workflow would be that the user establishes a certain balance eg. 100 USD and then uses this balance to purchase many small items/subscriptions/etc. The payments should be handled via automated payment gateways (Paypal or some credit card processor) or through manual payments added by admin.

Can you suggest any modules - ideally for D6, but feel free to post D7 solutions too - that would faciliate managing a user-based account in Drupal?

Btw, I am aware that the complete solution is not availible out of the box and that custom modules will be necessary.  I am also open to suggestions relying on external services or alternative ideas like the userpoints module.
Yes, Userpoints should work for this, with one major exception: It doesn't support decimal values. There are patches available for both D6 and D7 but you're pretty much on your own when using them.

There is integration with Ubercart available for D6 and Commerce for D7 (http://drupal.org/project/commerce_userpoints) which allows you to buy points with money. There are also modules available in the Userpoints Contrib project for requiring points to create nodes, which you can combine with Userpoints Nodes and Comments to remove points when a node is created, resulting in the ability to sell creation of nodes. And, there is Userpoints Node access, http://drupal.org/project/Userpoints_nodeaccess, which allows to buy access to nodes (only the D7 version works).

In general, the D6 versions of the modules aren't well maintained, especially the modules in Userpoints Contrib are very unstable (while those ported to D7 should work pretty fine).

Disclaimer: I am the Userpoints D7 maintainer and might be biased.