Dear P
Sorry I've not answered before - I've been away (France and Italy). You really need to hire me as your guide! I know Sicily very well and have visited nearly all the Greek sites on the island - most of which are much better than those in Greece itself. Most cities have online tourist/cultural info - and you can find them using www.google.com.But here is my insiders guide to some places you might overlook (working round the island from Messina southwards):

Naxos (below Taormina) - the first Greek colony on Sicily.
Megara Hyblaea (tucked away in the middle of an oill refinery between
Catania and Syracuse.
Syracuse itself - the cathedral still has the original Greek columns inside!
Museum is SUPERB.
Eloro - hidden away on the coast south of Syracuse - nice beach too!
Aidone (Morgantina) - stunning Greek city.
Palazzolo Acreide (fantastic Greek theatre etc)
Gela - the amazing city walls on the beach to the west of the town - and the
ancient remains overlooking the oil refinery. Gela itself is unpleasant.
Agrigento - of course: try to get there REALLY early in the morning before
it's overrun with tourists. The Museum is excellent. Also there was a temple
to Athena at the very top of the modern town.
Heraclea Minoa (south coast)
Selinunte - of course! But don't fail to see the Rocce di Cusa where they
massive column drums were still being quarried when the Carthaginians
struck.
Segesta ( of course) - superb views from the theatre.
Island of Motya (Mozia) - near Trapani - Cathaginian remains but wonderful
Greek stuff in the Museum
Palermo Museum is a must
Himera - nice museum and site
Tyndaris - mostly of Roman period - this was the last Greek city to be
founded in Sicily.
Also make sure you cross to Reggio di Calabria from Messina (ancient
Rhegion) to see the Riace bronzes.
AND - promise me this - you must make the trip to the Aeolian Islands (Isole
Eolie) - esp Lipari - Greek remains and an excellent museum (ferry from
Milazzo near Tyndaris).

The modern Greek connection is at a picturesque village between Palermo and
the infamous Corleone called either Piana dei Greci or sometimes Piana
degli Albanesi (Sicilians couldn't decide whether they were Greeks or Albanians!).
There are numerous Greek orthodox connections - many of the great cathedrals
(Monreale, Palermo, Cefalu) were joint labours of Arabs, Greeks and Normans. There's an excellent introductory book callled The Greeks in the West by
Woodhead (Thames and Hudson) - though it's probably o.o.p. Have a fantastic trip - prepare to be amazed!
Andrew