Can I run 2 power supplies on my pc?

I was wondering since I have little space for a larger power supply. Can I run the stock power supply in my pc for everything and tag on a separate, external power supply for my graphics card only?

The newb in me says "sure, why not?" Just put the extra supply on top of my PC and run the wire to my Geforce. My case is cramped anyway and I'd enjoy the cooling. I have a prebuilt HP, btw.

The wiser side of me says, "Wait a minute. Before you go and fry everything, ask the experts here." Ta da, here I am.

See I have no clue how PSUs work. I have like a 400 watt Stock PSU which runs my mobo and all my drives fine. I have no problem spending $80 for a new 600w PSU, but it's just a question of space. I really don't think I can fit a larger PSU in this machine.

So can I just put a new, shiny (even if I don't need this much wattage) 600W PSU on top of my case and run the PCI-E wire to say... a GForce 8800? If I just turn it on will it work?

I did some searching and found an ATX power supply will not work unless it's plugged into a motherboard. But a paper clip can be used to fool the PSU. Hmmm.... what are the pitfalls?

One of the problems with prebuilt HPs is that the cases are usually very cramped. I'd recommend replacing your current PSU with a Corsair 620HX but you have to make sure first that it fits inside. Same for the 8800 GTX, if that's what you're looking at, it most likely won't fit in the HP case. You can just buy a Centurion 534 case if necessary.