Looking for a new graphics card for my computer, will it damage it?

hi noob here, current rig:Intel® Core 2 Quad Q8200(2.33GHz, 4MB Cache),Vista,4GB DDR2 memory,640GB SATA hard drive (7200rpm)

1GB NVIDIA® GeForce GT120 graphics

the GPU is the problem

ive been looking to upgrade to nvidia, either the geforce gts 250 512mb or the geforce 9800 gtx. im just slightly concerned whether my rig would be able to handle it i.e PSU, motherboard.

cookies for anyone who can help, with graphics cards or even telling me what my comp with its new GPU would run. ciao

(glitchspot)

Open your case up and look for a sticker something like this.

whats this?jack_michael