I currently have in my Medion PC a Swissbit 1Gb PC2-4200U-444 and then some other digits. I was wondering if the manufacturers were interchangeable if you got the rest the same, and if you needed to get the following digits right also.

Brand names usually do not matter. Matching the existing chip specs does matter.

If you know who made your board you can check their web site for a list of compatiable dimms.

Also have a look at eshops to compare and ask suggestion. As rule professionals working there