Force closes are usually the result of an incomplete wipe, a bad download, or incompatible gapps.
You'll need to have /data, /system, /dalvik cache /and cache wiped in order for the flash to be considered a "clean" wipe. These partitions can be named different things depending on the recovery, but most have an option to wipe all of these for you, or have you manually do each wipe prior to a ROM flash.
Most ROMS will have an "MD5" posted along with the download. This is a string of numbers and letters that you can compare with the file you downloaded to confirm its the exact file that person uploaded.
I think this is the program I use:
MD5 Checker - Free download and software reviews - CNET Download.com
If the MD5's don't match, you'll have issues.