well i have pretty much the system that you are talking about i have (i5 660/ati 5770/4gb.mushkinram.1666/2intel x25-m 80gb ssd in raid0/gygabitep55a-ud3/e.c.o water cooler/
now if i could change i would go for the h55 mother board cuz it will unlock the new intel graffics..google read up on it.. h55/p55
now as for the gpu go for the 5770 and crossfire it.. the benchmarks of the 5770 in crossfire kill the ati 5850. and it is half the price so works out to be just about the same price if not cheaper.. but with better all around preformance
im getting mine in crossfire this week so if you can wait i will tell you how it gos.
and if you dont overclock but want to gygabite mother boards come with a windows base overclocking program that works stable. i know some people say dont use it but they have come a long way.
i have been useing it 4 months no problem went from 3.33GHz to 3.9 GHz small but very effective in fsx.