You can easily do this.  What you'll probably want to do is buy a new drive and a new caddy for it, so you can swap the drives with the removal of a single screw.  Without a second caddy, you can still swap drives, but it will take more time - undoing more screws and a connector, and reversing the process every time you want to swap the drives back.

The caddies cost about $15-20 - try Dell spare parts (call) or www.parts-people.com.

You can easily buy a 40G drive for less than what Dell is quoting for a 30 -- try www.newegg.com.

 

Thanks for the links: I got a drive form newegg, but ended up getting the caddy off ebay.  Thanks for the caddy tip too, I would have just done it the hard way had you not suggested it, and with the frequency that I plan on swapping drives I will certainly need it.