Since nobody has responded...
There is always a chance for an improvement by using HDMI or upconverting DVD players. Here's the kicker though: There is almost NO way to tell beforehand if there will be any noticable improvment.
There are three ways things could go...
1. HDMI looks significantly better than component video
2. HDMI looks almost exactly the same as component video
3. HDMI looks worse than component video
In about 95% (or more) of the cases HDMI looks the same or better than component video. In some, very rare instances, HDMI looks worse than component. I would say it is about a 50/50 split between it looking the same or better with HDMI.
Now, upconverting players - NOT ALL ARE CREATED EQUAL - may be a much bigger performance enhancer to your system. Many people think that it is the HDMI connection when it in fact a better DVD player that is giving them an increase in overall system performance. This is always a good thing for your system, but results are simply impossible to put a number on.
SIGNIFICANT IMPROVEMENT? No, but odds are good for some level of improvement. I personally have a 2+ year old Sony DVD player that I have no intention of replacing with a HDMI counterpart right now. I think the real money is on one of the HD disc formats and that, for me, means saving my money for a PS3 & Blu-ray which WILL be a significant quality jump over DVD.