You can fit a hard driver in the bay directly across from the X1800XT, if that's what you're wondering.
Mind you, I had to slide the HD to the front of the case to get all the cables in, then use one hand to hold the HD back in position against my X1900XT while screwing it in tight. As long as you have 90 degree (or rather flexible) SATA connectors you'll be fine doing that.