I'm going to suggest that you don't delete it since the 'Memory' area of any machine is generally a bad place to mess around with if you don't know what you're doing.
I suspect it will delete everything IF it is the only Memory item for Halo Reach, but you'll be able to download the Spartan again from Xbox LIVE. Maybe from recovering, maybe if you simply try to play Reach online.
You well lose all your campaign progress. But well keep your Halo Reach Rank and everything else. Your rank and what not is stored on the server. I play on two different 360's have one in my room and one in the living room. so every time I switch xbox's
cause I do not have a memory stick yet. I just recover my gamertag and my rank and everything is still there. However my campaign progresses are different. That is how I know.