First of all, it is important to check whether your power supply can handle it and that it can fit on your motherboard.
The 9400gt is a PCI-e card. This means it needs a specific slot to fit it onto your motherboard. I believe that the 6150e (never heard of it before) is a AGP card. This means that your motherboard probably doesn’t have the requirements to fit it in.
I get the idea that you want to improve your computer so that you can game better in the future. Keep in mind that gaming is not just the job of your graphics card, but also your processor and RAM.
Are you running on an Intel Pentium 4 chip or a Core 2 Duo ?
I don’t think it will have the chance to blow up your computer since it lacks sufficient power in order to run at maximum efficiency. You should make sure that your graphics card has fits into the type of PCI-e slot that your motherboard is using.