Consulting with your girlfriend is a good idea, but keep in mind she'll want you to be yourself and to be comfortable, so she might pull a couple of punches. Some of the problems people are dealing with are regarding "artsy" people, which doesn't precisely describe the design crowd. You will want to:
1) Be comfortable/yourself. Designers are all about individuality, so your personality shouldn't be stifled by something that retards your confidence.
2) Don't go with "artsy" stereotypes (turtleneck under a blazer, I'm looking in your direction), because this, if it isn't obviously YOUR look, might be considered the look of a rube, or even worse, a rude misconception of what is socially acceptable in "art circles".
3) Don't under-dress because of points 1 and 2. It will be in a gallery, and your girlfriend will be exhibiting, so dress up to her level. Don't, however, dress to the point of outshining.
You will most likely see all manner of dress there, from people who just drop into the gallery (very casual dress), to designers and design enthusiasts (usually very cleanly put together, but also casual. Look for radically sporty PUMA sneaks), to partners in design firms (suits/blazer slack combos, and open collared shirts). That said, if it puts you at ease at all, there will be very little standard to follow. The exhibitors are usually happier if you step it up a bit, though.