Sealing the walls only works reliably from the outside. My parent's basement always flooded with rains like we've had the past few days, and really bad with Floyd in '99. Instead of digging down to the foundation across the back, I went out about 12' from the house (to avoid patio/etc.) and put in a french drain/curtain drain across the entire width of the back of the house. 2 4" pipes, 7' deep at the deepest end, draining to daylight. I forget how many tons of gravel went in, but it is gravel and stone from the pipe all the way up to the surface, so it also catches ground runoff.
In a heavy rain, the 4" output runs like a firehose. 
I'd try that first, cheapest and easiest, a couple or three days work with a rental backhoe. (I rented a terramite after my ancient trencher/backhoe broke the crank in the middle of the job. 
