both lonewolfarcher and bobdammit are both more knowledgeable than i am, but i'll give you my 2 cents worth. first, if you don't have astro installed yet,get it from the market (free). open astro, hit menu and select tools. one of your options will be application mamager/backup, select that and from there you're all set. just pick whatever apps you want stored on your sd card. after you've wiped your phone, just go back into astro and do the same thing, but this time you can choose which apps you want to reinstall on the phone.
now as far as telling you which rom you should choose, that's strictly personal choice.
that would be like telling you ice cream had to be your favorite dessert. maybe it's really good,but i like chocolate pie. roms are like that, all good, just depends on what you like better. personally, i'm a huge fan of xtrrom. love the stability. here's a link to most of the roms from xda. just do your homework and you'll find what's best for you. [READ FIRST] Eris Roll-up Thread - xda-developers
The only thing mamawm said here that I disagree with him on is that I am more knowledgeable than he is. This is dead on advice!