My answer is NO, DON'T DO THAT.
Overtaxing a power supply is shaky ground and at least here not very expensive to fix. See those 500+ Watt PSUs at geeks.com. I use the 680W units since there is no need to go lower and the PC will use what it needs so the extra is just headroom. We want a lot.
I recently upgraded to XFX Nvidia Geforce 8400GS. The XFX Recommended Power supply requirements written on the cover box is 350 Watts, But the power supply of my PC is 300 Watts. The graphic has been correctly recognised by my motherboard and it's working OK.
Also ever since i've installed the Geforce 8400GS card, the IEEE 1394 Adaptor(FireWire) component has been disappeared from the Network Connections.
Will there be any performance drop in the graphic card, will this card damage the motherboard or any other PC components and why did the IEEE 1394 disappear ?
Graphic Card :- XFX Geforce 8400GS 256MB ON BOARD SUPPORTING 512MB DDR2 (Model-PV-T86S-WAQG)
Intel 945G Chipset
Intel Pentium 4 3.06Ghz HT
2 GB DDR2-533 Dual-Channel Ram
80 GB Hard Disk
Windows XP Home