I'd say stick to the old 360. It will give you better gaming performance than your laptop.
Here's how the 425m ranks relative to other video cards from a performance standpoint:http://www.videocardbenchmark.net/high_end_gpus.html
It's more geared toward multimedia and productivity.
The 360 will do a better job with gaming.
Most laptop graphics cards aren't interchangeable, but you could contact ASUS to find out if they have any compatible upgrades and if this is possible with your particular model.