CS3 is actually pretty good at interpolating images up to larger sizes. Use the bicubic softer option and it should be ok.
How big is the original image and what size are you aiming for? It may be worth looking at what's called 'stair or step interpolation,where you increase in 10% steps until you get your desired size.
Are these for printing? If so, dependant onthe resolution of the printer you may get away with not interpolating at all. I had a 30x20 print from my d70 via photobox and it came out fine.