I agree with Hawks... You can pickup a old Option 11C system fairly cheaply and still use your Taurus's. The main cost of any system is going to be the end-points, so if you go with another solution you are probably looking at $120,000+ on the low-end, just to replace your endpoints.
However, I would investigate how good the ACD is in the system and see if anyone on an asterisk based system is running a 800+ seat callcenter.