8600gt and power supply upgrade

Hi, I'm trying to build off of what I have to make a midrange gaming pc. this is my computer right now

http://www.circuitcity.com/ssm/Spe [...] on.do#tabs

intel core 2 duo e6300
1 gig 533mhz
integrated graphics
250 watt hp psu

I did not build this i bought it. it has an open PCI express 16 slot that I would like to put a 8600gt into. here's the link to the specific one i want.

http://www.newegg.com/Product/Prod [...] 6814122026

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Now, the system requirements state that you need a 350 watt power supply with 12V and 20A. forgive me for being a noob but I honestly don't know if what kind of power supply I need BECAUSE I HAVE A HP computer (a1720n) and i opened up the case, the power supply seems like it's not a normal/standard ATX power supply. I'm not 100 PERCENT sure but my guess is i have a Micro ATX power supply because most of compaq, HP, gateway ect. use micro atx. the psu measures 6 inches deep, 3-4 inches tall, 4 inches wide, seems very small. I tried to find a MICRO ATX power supply that has good wattage but most of them are very low. I've only found these 3:

http://www.tigerdirect.com/applica [...] CatId=1078

http://www.tigerdirect.com/applica [...] CatId=1078

http://www.newegg.com/Product/Prod [...] 6817338008

I know this is enough for the 350 watt they state but i don't know if the voltage or amps 12V 20A or whatever is enough. someone please tell me if this is more than enough to run the 8600gt. This will be my first time trying to install help would be appreciated, if you know a Micro ATX power supply that can run the 8600gt please post a link.