All present GSM networks (1991-2001) and all GSM phones are using GMSK modulation. EDGE uses 8PSK modulation. 

EDGE is a modulation scheme. EDGE enhances the number of bits that can be fit into a single radio channel. GPRS provides a packet access protocol for the GSM system. So, wherever you get EDGE coverage, with an EDGE enabled phone (and you won't be able to upgrade existing ones - typically they need a new radio), you should get 2-3 times the data throughput you did before. GPRS simply gives you the 'always on' connection. Think of EDGE as the equivalent of moving from a 19.2 Kbit/s modem to a 56 Kbit/s modem and GPRS as the equivalent of moving from a modem connection to a network connection.

Finally, the reason you don't get 117 KBit/s on existing GPRS is partly the phone capabilities (lots of DSP power required = flat battery very quickly) and partly the spectrum requirements - a 117 KBit/s connection will use 8 times the capacity of a speech call for the duration of the transfer. Hence the attractiveness of EDGE as a panacea. However, just as the industry says they can get 117 Kbit/s out of a GPRS system without EDGE but only manage 43 Kbit/s, the 473 Kbit/s figure is more likely to be 150 Kbit/s or so. Of course, with 3G, the data rates will go up again and will be packet based from the start. Look for 384 Kbit/s as the first level of service there, but only in urban environments. Phones will be dual mode and revert to GPRS outside 3G coverage areas.