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.

I run two psu's, a 550 watt TT Toughpower, for the mainboard and such, and a TT Powerexpress 250 watt psu in a 5.25" drive bay for my 8800 GTX.

Rock solid 24/7 delivery with a e6750 (oc'd to 3.4), 2 hd's, 2 dvd/rw's, x-fi, and a swiftech watercooling system for my rig. 3 120 mm fans round out the power suck, along with a few usb devices.

The drive bay psu was under $50 almost anywhere, and runs cool and quiet. Loudest thing I have is my 680i NB fan at 50%.

Money well spent on the drive bay psu, has 2 6 pin connectors.

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?

Wow, surprised no one got back to you on this one... I would say, that a QUALITY 600 watt should run your whole system fine... You could just dump the 400 all together. What are your system specs and what is the PSU you have right now?

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.