New Build: Q9400 or 810 or 910

I would get the Corsair 650TX. It's the best 650W PSU ever AFAIK (except maybe the Antec Signature 650W which cost twice as much).

The ASUS M4A78T-E/3 GB of DDR3/X4 810 would cost you $377 in total. You can do better:

Phenom II 955 + GA-MA790X-UD4P, $325
http://www.newegg.com/Product/ComboDealDetails.aspx?ItemList=Combo.190735

G.SKILL 4GB (2 x 2GB) 240-Pin DDR2 SDRAM DDR2 1066 $59
http://www.newegg.com/Product/Product.aspx?Item=N82E16820231166

That is, for $384 instead of $377 you can have a seriously better CPU and 4 GB of RAM instead of 3, and the MB is still high-quality.

The Q9400 build comes to 145+220+57 for a total of $422, and it's weaker than the $384 Phenom build and has only 3 GB of RAM. I think Intel needs to cut prices on the Q9400 fast...

The HD 4890 is OK. It will even support Havok effects. For example, that scene in the Diablo 3 demo where the barbarian drops a wall on top of a crowd of zombies  

I wouldn't bother with the Xigmatek. If you don't overclock, the stock cooler is just as quiet and it doesn't matter that CPU temperatures will be 5 or 10 degrees higher.
Besides, I have no idea if it fits in your case since you didn't say which case you've got.