Id go with option 3 for sure. The motherboards in those factory systems are crap and you cant oc them. Why get an I7 that you cant overclock? Also dominator is really good ram. that factory stuff in the gateway doesnt even have a heatsink on it and isnt good for gaming. plus its 1333mhz. you want 1600. I would go with the custom build but see if you can get the ASUS P6T or EVGA motherboard. The MSI is good but not as good.
I think $1700 for that system isnt expesnive. Think about it
I7 = 300
Antec = 100
HDD = 100 - Caviar blacks only 10% worse than VELOCIRAPTOR!
Dominator = 300 - maybe get cheaper ram to decrease price?
Power supply = 150 Good choice! Corsair PSU is awesome!
GTX 280 = 350 -- what about the 285?
Mainboard = 250 for the MSI Platinum
Sound card and DVD player = 100
Total: $1650 Thats a good deal if only cost 50 to build and overclock.