1. there's a couple of guides here... but it's really easy to setup in amon.
- first, copy all your data from SD to your PC (i.e., backup your data)
- boot into recovery
- scroll down to "Partition sdcard" - press trackball to confirm
- for "swap-size" scroll down on trackball to choose "0" (this may be were you hear "lag" from... our internal Eris memory is quicker than your SD for swap)
- for "ext2-size" choose how much space you want for your apps... i choose 768mb (but this is quite alot... for most 512 is plenty)
- "fat32-size" will be the remainder of space on your sdcard - press trackball to confirm your settings and start partitioning.
- once done... you go back to the "Partition sdcard" menu and choose "SD: ext2 to ext3" - press trackball to confirm
after this, your card is ready to go... you can copy your data back.
as for "setting up" on roms... to be honest, in my experience, it varies. some roms work GREAT w/apps2sd... others take a lot of "playing" to get working. both cyanogen based roms here worked great with apps2sd... and it was as easy as going to Settings > Applications and checking the apps2sd box. reboot, install apps, done!
2. i'd say most apps will run just fine from the stock sd card. i didn't notice any lag when using mine (though i upgraded to a class 4 16gb card that i got on the cheap from ebay). again, if your swap was on the stock sd, it may cause lag (and i wouldn't run the swap on sd unless i had a class 6 sdcard, personally)
3. again, this will vary from rom to rom. when i switch between the cyanogen roms, my apps setup/restore just fine