Then Switch off the phone, switch it back on holding home key & power key, to get into recovery mode.
To enable Apps2SD (install apps to sd card) you must create a partition on the SD Card. To do this click on partition sdcard. We want 512MB as EX2 and the rest as FAT. After this has completed, go back to the partition SD menu and click on SD:ex2 to ext3
WARNING: you may or may not have to re-flash your phone. I've never partitioned the phone after installing a rom, I've always partitioned the sd before.
Maybe wait for someone to comment on if it does require reflashing beforehand?
Apart from that, thats the only risk.
nope, you arent messing with the bios anymore, if the phone is already rooted. If you think of it like a pc, you are just changing/fiddling with the Operating system.Which as everyone knows, one can just wipe and re-install and everything is fine.