what else will you be using your rig for?

if you'll also do gaming and work on it, id go for two gpus instead of a single 5970.
You can use the 5450 as main gpu for adobe photoshop or watching movies while the 5970 does the mining.

or get two HD5870s. The two can get 600 mhash/s if they will both mine.
But you can play games on one while the other does the mining.