CLT Controller described a loss of separation event between an arrival and departure on Runway 36C; noting questionable spacing judgment and 'rushing' as factors that lead to the separation loss.

Narrative

I just relieved a controller from the Local Control Center (T) position. As a heavy B767 crossed the threshold cleared to land; I instructed a B717 to 'line up and wait be ready to go' on Runway 36C. As I gave the B717 those instructions; another Air Carrier was just inside of 4 NM final cleared to land Runway 36C. I instructed the heavy B767 to exit the runway at Taxiway E8 without delay; which he complied. As the heavy B767 was turning to exit the runway at the high speed Taxiway E8; I quickly instructed the B717 to be ready for an immediate departure; which he acknowledged. When the heavy B767 safely exited the runway; I cleared the B717 for an immediate takeoff. The B717 was very slow on his departure roll. As the B717 started rolling down the runway; the arriving Air Carrier was closing in on final to Runway 36C. As I realized the aircraft may be closing in on an unsafe proximity; I alerted the other controllers in the Tower of my situation and that I may possibly have a go-around. When the arriving Air Carrier was short final; I keyed the frequency to instruct him to go-around; but said the wrong call sign in the go-around instructions. At that same time; the B717 was becoming airborne. About 2-3 seconds went by when I realized my mistake; and I keyed up the frequency again and gave the go-around instruction to the arriving Air Carrier. The arriving Air Carrier replied to the instruction by advising that it was too late for him to go-around and that he was landing the aircraft. When briefed it was determined that separation was lost and the aircraft involved were separated by an estimate of 4;500 FT. The only recommendation I have is for myself to use better judgment when determining an appropriate departure interval and not 'rushing' to clear departures for takeoff.