Permalink question

Ok, I have read everything I can about permalink switching and nothing has really been right on point to help me, so now I’m coming here to ask exactly what I need answered.

I have tried everything I can to switch my permalinks from the default setup to %postname%. When I do it through the wordpress permalinks options, and then go to my site all my old posts point to the new permalink structure but come up as 500 Internal Server Errors. So I have to switch back for anything other than my homepage to work. I downloaded the permalink redirect plugin, but you have to put in your old permalink structure for it to work (ie %category%, %postname%, etc.) yet I cannot find the structure tag for the default. I don’t even necessarily know this would work. Then I read all this stuff about the .htaccess file, and I checked mine and all the code in it is correct.

I’m trying to change this so I can make the Flickr Photo Album plugin work, it won’t work with default permalinks, but the fact of the matter is, there is just WAY TOO MUCH info out there, everything pointing in different directions, and I’m overwhelmed. I’m sure there’s a blatantly simple solution to all of this and I cannot just figure it out.

Thanks in advance for your help, it’s much appreciated.