2010? - I5 gaming rig - $1700

First of all, the motherboard: i really like Gigabyte they have less problems than ASUS and are most reliable but if i were you i wud forget the UD3 because while you wont be doing XFire its always a good idea to leave that option open so i suggest you go for the UD4 so you will have 2x PCI-E16x II lanes whereas the UD3 only give 1 16x PCI-E II & 1 4x PCI-E I and the UD4 is really a UD6 missing some of the embellishments.

Graffix card: i too like the idea of getting on board ATI`s new road show by buying a reasonably priced 5xxx series card but the card youve picked is similarly priced to the 4870 and 1or2 4890 cards both of which are faster but i wud go for the 4890 myself the main reason being that it can handle the high resolution of the screen youve chosen whereas the 5770 is going to struggle with its 128bit memory bus.

PSU: thinking about that XFire option shud you decide to go there in future you will need a unit that has 4x PCi 6/8 pin connectors and the Corsair TX 750w fills that bracket perfectly and is prolly the best value/performance ratio around.

Computer Case: although the Antec 900 is a good case its a bit dated now and the thing i dont like about Antecs is the crumby cable management, its non existent unless you mod the case yourself. A lot of these problems stem from having to mount the PSU upside down which Antec recommend you do and that puts the wiring on the wrong side for management which is all important for that airflow and heat dissipation. Ive got a CM Storm Sniper which is only `a few dollars more`and gives you looks, cable management, a multi fan control centre, filtered intake, 3x200mm fans + a 120 at the back and its modern and beautiful.

http://www.newegg.com/Product/Product.aspx?Item=N82E16811119194
http://www.newegg.com/Product/Product.aspx?Item=N82E16817139006
http://www.newegg.com/Product/Product.aspx?Item=N82E16813128400
http://www.newegg.com/Product/Product.aspx?Item=N82E16814102848

Have a look at some other 4890 cards before you decide the Saphire is in the middle price wise, the rest of your stuff looks ok to me but do consider what ive said