Graphic card for 350w PSU

Ok, so I have a new Dell pc (before someone bashes, I got it really cheap online, much much less than making my own.) I was relying on using my old 7900 gs to power this thing for games... but now I find that the 7900 isn't working.
This PC has a 350w (although I'm suspicious that it only has 300w) psu and I need a gpu that will be decent for games, like Company of Heroes or Supreme Commander, but one that will work with my 350-300w psu.
I know how to assemble PC's from scratch, but have gotten turned off on the idea of removing the PSU on this one. I'm impatient with the hardware side of things. Replacing it is just out of the question for now.
So,
What's a good GPU which will work with a 350-300w PSU?
Do you know of any external power supplies? You know, so I can just plug a GPU on it without changing the internal PSU? I've seen one of these in fry's but I didn't need it at the time.
What would happen if I put in a video card that my PSU couldn't handle? Explosion? Broken computer?
Thanks in advance, good day

Pc specs:
q6600
6gb ram
Vista 64
250gb sata

To answer myself here, I'm partial to the 9 series videocards from Nvidia. Particularly the 9500gt: (from their website)

Maximum GPU Tempurature (in C) 105 C
Maximum Graphics Card Power (W) 50 W
Minimum System Power Requirement (W) 350 W

Is the 350 minimum taking into account that I have a powerhogging q6600?

Yes that looks good. Haven't considered or used ATI before. Is that better than the 9500 or 9600gt?

Ok thanks. So what's the worst thing that could happen if it isn't 'compatible' with my PSU? It won't break stuff wouldn't it?

Another vote for the HD 4670, wayyyy better than that 9500GT, and takes ~20 watts less power than the 9600GT.

Well, if it had a 7900GS it should be just fine with a Radeon 4670. That Radeon 4670 will be a big improvement. The 7900 GS draws a max of around 80W I believe, so as long as you don't get a card that draws more than that you should be fine.

i would say visiontek they have life time warrenty

HD4670 has more goodies too... HDMI ... no need audio connections from outside the card

I have an MSI 4670 with a non-reference cooler I really like.

Good choice of cards for a prebuilt case. You'll have to look at your motherboard and the PCIex16 slot. If it's a BTX build you won't be able to use the slot above it. ATX build, the slot below it. Measure and make sure it'll fit.

Did you mean HIS? I like that cooler too. I bought that style of HIS card for my father's non-gamer build, but a 2600PRO. It was a lot quieter than it looks, too. That's the one I'd get if I bought a 4670.

From what I understand the Dell psu's are underrated and if you don't overclock or add additional peripherals the 30amp 12v rail on that thing can power a 9800 gtx with ample head room. Don't go adding any higher cpu though, you'll want to keep you total system draw around 200-250 watts at full load. The rating system on dell psu's reflects actual constant wattage available, if it say 350w then much like shuttle psu's you get 350w out of it. Unlike consumer psu's dells rating is for part specification not to make sales. But give yourself the 100 watt head room and make sure that your 30 amp rail gives you ample head room, I believe a 9800 will draw 9-12 amps. People also run 4850s too.

Warning this is not a guide to a consumer PSU. Just Dell's and shuttles.

Anyone who is suspect of this should be aware that the the 12v rail on this is ample and provides 300 watts constant on the rail alone.