what exactly models are your harddrives? and if they're seagate, when did you buy them?
as for RAID, which type of RAID are you thinking of setting up, or which raid configuration did your friend suggest? with 2 drives, your options would be raid 0 or raid 1. I'm assuming your friend is suggesting raid 0 for extra performance. but know that if you do this, your data would be stripped between TWO harddrives, with neither drive containing a FULL COPY of your data. thus if either of the drives fail, you lose everything. as for RAID 1, both drives are mirrored, so you have TWO copies of your data, but you don't gain anything in performance, you just gain security at the expense of harddrive space.
for starters, BACK UP all your data to an external drive. raid would reformat all the drives so both of your current HDDs will be wiped clean.