According to the reviews, GH1 provides better quality in higher ISO than G1.
Luminous-landscape.com
Changes From the G1
The following is my report on the Lumix GH1 considering it primarily as a video camera. As a still camera it is little changed from what I wrote about in my initial review. The sensor is new, though still effectively 12MP. This allows the camera to shoot various still aspect ratios, from 4:3 to 3:2 to 16:9, all with the same coverage angle. The new sensor also seems to have given the camera somewhere between a half stop and a full stop of extra ISO, at last in terms of low noise. ISO 800 on the G1 was acceptable but not great. Now on the GH1 it is on a par with cameras in the APS-C sized sensor category from other manufacturers, and ISO 1600 is acceptable, which before it was not.
That's about it for upgrades from the G1, so if you'd like to learn more about the still photography side of the GH1 please read my original G1 review and then come back here to learn about the video side of things.
dcresource.com
The GH1's photo quality was very good. Photos were generally well-exposed, though it did underexpose by a third of a stop on more than a few occasions. Colors were vivid -- no complaints there. The original DMC-G1 was a little soft, and the GH1 seems a bit worse, though I think the kit lens has something to do with that. If you agree, it may be worth turning up the in-camera sharpening a notch or two. While it's not quite as good as the best D-SLRs in terms of noise, the DMC-GH1 is competitive, and a bit better than the G1. You can safely shoot through ISO 400 in low light and ISO 800 in good light -- and shooting RAW will let you go a little higher. Panasonic cameras automatically remove purple fringing, so that wasn't an issue. I did spot some highlight clipping here and there, but it wasn't major. Redeye levels were mild.
I ordered an GH1, can't wait to have it in my hands.