Essentially, other factors will intervene long before the molecules or atoms of a substance will reach the speed of light. Besides, heating has more of an affect on density and coherence than it does speed.

First off, the solid state of a substance is basically its frozen state whether it be water, which solidifies below its melting point of 0°C (32°F) or titanium, which solidifies below its melting point of 1668°C (3034°F). Below the melting point, a substance's molecules are more tightly packed and slower moving and form some kind of solid structure.

Above the melting point, the molecules are more fluid (hence the name) and do not form solid structures thus the substance is a liquid. here though the temperature plays a factor along with density and the melting point. some substances such as water have a low viscosity, which is a way of expressing how fluid it is and how well the molecules move around each other (hence why water can be very fast moving in rivers and streams), and others such as iron have a high viscosity (if you have seen molten iron, it is thick and slow moving).

As a liquid is heated, the molecules move farther and farther apart until the temperature reaches the boiling point (100°C or 212°F for water, 3287°C or 5949°F for titanium) where the molecules can no longer maintain enough connection and the substance turns to vapor.

Now, what happens next as you heat a substance? Plasma. Essentially, as you heat a gas, the molecules will disassociate themselves and breakdown into the component atoms. As you continue to heat these atoms, they begin to ionize or loose their electrons. Plasma is a state of matter much like a gas, but where some of the atoms are ionized. Plasma contains a mixture of positively charged ions and negatively charged electrons. Plasma has properties that are far different than those of solids, liquids, and gases; it is essentially a distinct state of matter. Plasma does not possess a shape or volume unless under the influence of a magnetic field where it forms structures. Stars are giant balls of plasma held together by gravity and magnet forces. Our sun is a great place to see examples of the types of structures that form from Plasma under the influence of a magnet field... the giant filaments that reach out and come back to the surface for example. Another example close to home of a plasma would be what happens to neon gas when heated in a neon sign.

What happens next? The speed of light. As the atoms of a sun burn in a fusion reaction, they result is a burst of energy in the form of sunlight, neutrinos, and other forms of electromagnetic radiation.

The way I see it is, that if you were able to get it hot enough, to the point the molecules were at the speed of light, or even surpassed the speed of light-----you would find the way to time travel. Theoretically, you could go back in time and see the images and events that were going to happen, once the light caught up to you.

In another logical sense, we know that no substance that has been found on earth or made has an infinite melting point. There is a point at which an element is heated, that yes the molecules would move faster and faster, but its not that they would stop heating, I think they would fizzle out.....but then again, that goes against the theorem that matter is neither created nor destroyed. Right?

I'm sticking with my time travel theorem to this. lol

Here is a pretty cool website, and I don't want to say things from it and use them as my own ideas, so I will include the URL so you can read yourself. I don't want to take credit, and this has some neat ideas in it.
http://www.wisegeek.com/what-is-the-highest-possible-temperature.htm

The Maximal possible temperature that a substance can reach is called the boiling point.

Matter --Heat till liquid-->Melting Point --heat till gas-->Boiling point. Evaporation.

The problem with your question is that you are trying to use simple, old English terms to describe situations beyond the limits where they are useful. In particular, what is a "substance" in a context like this? Also, what is "heated up" in this context? You can put molecules in a particle accelerator and speed them up to near light speed; but, are they really hot? The word is not really useful to describe that situation.

The hottest place where one uses temperature in the conventional (more or less) sense is in the core of stars. The hottest I have found mentioned is 1.1 gigakelvins (1,100,000,000 degrees Kelvin) but this isn't the steady temperature of something you can stick a thermometer into, it is an ongoing nuclear reaction where individual atoms don't last long enough to really have a speed, let alone the ability to form a molecule.