You may need a 245 to stretch across the 8.5 rim, but you'll have to watch for clearance issues with the strut (the part supporting the bottom of the spring, not the side of the strut). I had a 245/40/18 on my 01S and there was only the thickness of 2 credit cards between the tire and strut. Certainly a 35 series tire will be lower than a 40, but we're also talking 19s vs 18s so you lose whatever advantage you had going with the 35. I'd check with a tire dealer and see if they know the overall height of a 245/40/18. Wouldn't want your 19 to exceed that height.
Using the handy-dandy wheel offset calculator and comparing your 19s to std 18" Boxster wheels, you get the following:
front: 7.5x18 50mm --> 8.5x19 56mm 19mm extended in than std, 7mm further out than std
rear: 8X18 50mm --> 9.5x19 45mm 14mm extended in, 24mm further out
http://1010tires.com/WheelOffsetCalculator.asp