To address the OP's question, no - adding RAM won't improve your ability to burn DVD's. If you have had speed issues with CDR, then there's a good chance you will also have speed issues with DVDR, but RAM won't help. Your HD is the culpret.
The single best upgrade you can do for an older system, for burning or anything else, is a new hard drive. Get a good drive with 8MB cache and re-install Windows to a clean drive. This will speed things up more than you can imagine, and free up loads of space while eliminating fragmentation. When you install Windows, set up the paging file to the same initial and max sizes, around 1 GB, before you install any programs. Keep the drive defragmented. If you plan to store many movies or large video files, a second drive for that purpose is highly recommended.