Why you unaccepted Dan's answer is beyond me, it is good sound advice. Perhaps he wasn't giving you the technical information you wanted. Many things are possible, that are not advisable. What you are wanting to do is beyond most techs' skill level, even if it were possible.
First Road Block: The 2 GPU's are based on different architecture. The ATI Radeon X600 XT (Native to your A1145) is based on the RV370 (110 nm process) Graphics Processor. The ATi Radeon HD 2400XT is based on the RV380 (130 nm Low-K process) GPU. NOTE: Graphic cards based off the Radeon X550 may interchange with your existing architecture., using the same RV370 chip. Some of what I have said can be confirmed here
2ND Road Block: Apple/Mac computers do not recognize a video card/chip unless it has the Apple firmware. Thus, even if you found a chip than physically can be attached to your card and still function, your A1145 will reject it without the chip being flashed with the proper firmware. If the chip is not properly flashed you will "brick" it. Wasting all your time putting the new chip on your video card. You are now back to square 1.
There is a site dedicated to people such as yourself that want and/or need to change the Graphic Processor in their Apple computer/laptop. Go here dig around. You might find what you are looking for without asking any questions. The site is very informative and the only place I know of to get custom Apple firmware for GPUs. It is very possible to find other video cards that will work in your computer that only need flashed to the Apple firmware to work in your A1145. If you don't find any mentioned there, ask about such.