Compression, eq, and reverb. A great screamer is always a plus 8-)

sm7 or sm58, and the sansamp plugin all come in very handy. as do compression and eq. don't overdo the compression however.

watch those red lights!

thanks alot to both replies!!

Yeah I would also go for the 58. Has a certain "je ne sais quoi" with screamers.

Also try the following trick: Double the final vokal take, and distort it with a plugin like trash, sans amp or amplitube. Then fade it in as necessary. Adds a grittiness and dirtyness that can make it very interesting. Maybe not for the whole track, but only during those real screams.

If they scream, stick to a dynamic mic like the SM57. If they scream like a vocal coach teaches you, then use a good condenser.