Don't expect a quantum leap but expect more of the way Nvidia conducts business, re-branding, add a few more shaders, raise/lower GPU cores/memory speeds - it's all towards market segmentation and they want a piece of the action on every level. When the 300 series finally arrive, it'll be the Bugatti Veyron or the Ferrari Enzo of the GPU world BUT in 12 months it becomes a mid range card!
How much better will it be than the GTX 295? A realistic guess for me would be a maximum of 8 percent performance gain (10% is just wishful thinking).
Instead of market hype about the latest and greatest because it's now GDDR 4 or with more SPs, I'd rather see NVidia make all their entry level and mid range cards "cheap dual-GPUs built in SLI on one PCB" by using, for example two old G80 or G84 chips. They'd save huge on the R&D and still come out with very competive products.