I used guiminer for a long time until I found cgminer. Switch now and avoid the regret of not doing it sooner.
You can switch pools on the fly, configure multiple backup pools, monitor temps, overclock, and a lot of other stuff, all with a simple .BAT file.
You only need to run one instance, and you can dedicate separate cards to its own pool. All kinds of cool shit.
I always had ~3% stales using guiminer. With cgminer, it's closer to ~0.3%. Yeah.