Yes, the added speed of the T7250 will be better than the 5450. As for the hard drive, I wouldn't be as concerned with that as I would the graphics card. Right now you have the GeForce 8400 selected and it's not a very game worthy card. If you can afford the GeForce 8600, I would step up to that as well.
As for the battery, it depends on how much time you want to spend away from an outlet. I read a review that showed the 9-cell as getting over 3 hours of battery life (3 1/2 I believe they said). I would imagine the 6 cell would get a little over 2 hours, but I can't say for sure.