What I did was replace the CGminer that comes with GUIminer with the 3.7.2 distribution. Then I used these setting at stock 947/1250 clocks on my 290's:
Intensity 20, Worksize 512, Thread-concurrency 27900, Thread 1
This gives me 840 Khash per card running at only 1.09V with -31mV through Afterburner. I feel it is a good tradeoff between max Khash and watts used.
Which GUIminer are you talking about? GUIminer or GUIminer scrypt-alpha?