I'm no expert, but I believe "mAH" stands for "milliamp hours", which is a measure of how long the battery will last under a given load. A 150mAH doesn't put out more power than a 120mAH battery, but it can hold a larger charge capacity, for longer battery life.

"A", or "amps/amperage" is a measure of electrical current. The amperage rating of 2.5A means that that particular power supply is rated to be used with equipment that will draw no more than 2.5 amps. Drawing higher than 2.5 amps will probably damage it.

{edit}
I guess I should have added:

You say that your current power supply is rated at 700mAH, (probably 700mA). I am very positive that charging a mini r/c car is not drawing anywhere near 2.5 amps, especially if your 700mA is working, so the one you were considering at Radio Shack is more than enough to handle the job....