It's not about viewable area as much as usable resolution. If you can get 1280x960 or 1280x1024 out of your 17" at a refresh rate high enough to keep your eyes from watering (85Hz for me), then a 6800GT won't disappoint. If you're stuck at 10x7 or 11x8, might as well save your money and limit yourself to a 6800 or 6600GT. Heck, even a plain 6600 that you OC the heck out of may be enough to tide you over.
Otherwise, the rumor is that ATi's next gen will hit around May/June. If it's 300+M transistors, as the Inq "reported" (*cough cough* 😉), then I'm expecting a big performance boost. Before that tranny count, sites like Hardware.fr were speculating at only a 30-50% performance hike. But 300+M transistors in June on 90nm low-k could make things interesting for nVidia.
Don't forget that a 6800GT appreciates read: may require) a good 350W PSU, so factor that into your price if you need to upgrade. The 6800 actually uses less power than a 9800P, and the 6600GT not much more, so you probably won't need to worry about your PSU with either.