case problem

[ferrarif401]

i have a gateway g6 400 computer and i'm planning on upgrading the mother board, cpu, power supply and ram. my question is can i just use the gateway case instead of buying a new case. i am planning on just buying a power supply and putting it in the gateway. another question, will the gateway power switch fit on the mother board. i'm planning on getting, asus a7n8x, amd athlon xp 2600+, a 350 power supply, and two sticks of 256mb ddr pc2700 for dual channel memory. but can i just keep my case and not but a new case? please help!!