what will a hit from a 200,000 metric ton projectile of various composition look like.
It depends on their speed, really, but since the thread title includes "relativistic", it would look like a white flash no matter the composition. Because whatever the composition, the projectile will break down to at least atomic particles on impact.
Also, 200,000 tons is a hell of a lot of mass to come flying at relativistic speed. Just flipping it over in my head, at 0.9 c that should be around 5.8e9 MT (about 5.8
billion mega-tons, in case you think I wrote that number wrong) of TNT. That should be enough for some six to seven [ame="http://en.wikipedia.org/wiki/Wilkes_Land_crater"]Wilkes craters[/ame]
The impact would certainly look spectacular, very bright (i.e. white to the naked eye) and should be visible a few solar systems away. The composition of the projectile certainly won't figure in what the flash looks like (unless you're looking at its residue through a spectrometer from a few ligtyears away, maybe).
Seriously, I think you should make the projectile less massive...
What will happen when these projectiles hit a planet with and without an atmosphere.
You wouldn't be able to discern anything unless you're watching the whole thing in ultra-bullet time. It's happening too fast. Presumably there would be a very devastating shockwave propagated through the atmosphere leveling pretty much every building on the planet (like a tsunamy, only that this time the wave is hot air instead of water), but most of the energy should still be absorbed by the body itself, which might completely tear up less dense bodies (not the earth, though). But even if the earth wouldn't fly to bits, you can bet that the resulting earthquake literally won't let a stone stand on the other on the whole planet. You'll have a very prominent landmark the size of a small continent on impact, some new mountains, probably a lot of older mountains gone, and I wouldn't be surprised if the even initiated a new continental shift.
I can't calculate by how much the crust would heat up, but it would be measurable (wich is scary enough).
Could the projectiles be seeded with radioactive materials?
Of course they could, but it would be utterly pointless. As I said, the projectile and the immediate surrounding of the impact would come appart at the atomic level. I think there would be quite some X-ray release, probably neutrons, and the whole impact area would glow green for quite a while...