If i had a 775 i'd go straight to an E8600. Yeah its a dual core but the I7 is expensive and it should easily hit 4.5Ghz and do it daily
A possible 4.5ghz, and it's not a given just a possible 4.5ghz does not warrant an extra £80 IMHO.
+1 4.5 ghz will require a very decent Mobo and Ram and Psu And Cooling with a fair deal of knowledge and a little luck.
what is the machine used for on a daily basis?