It might be easier to swap the old sdb into the sda position so you'll boot from it, but you should also be able to boot the install cd with
If swapped and booted, and got a kernel panic error.
What does it complain about?
<snip> mounting root filesystem cfs: can't find ext3 filesystem on dev md2 mount: error 22 mounting ext mount: error 22 mounting none switching to new root switchroot: mount failed: 22 unmount /initrd/dev failed: 2 Kernel panic - not syncing: attempted to kill init!