Hi guys.  A bud of mine just upgraded his hp a1130n w/ a xfx oc 8600gt w/ zalman fan, and 2 more gigs of ram (total 3gb now).  His psu is stock 300watts.  He has a dvd-rw and cd rom drive, media card reader, 250gb hdd, amd 64 3500+.  He plays on 1280X764 reso.
 The min requirement for 8600gt is 300watts (via tiger direct); however, his card's overclocked.  When playing company of heroes ( not sure if only coh);  after a while the computer restarts or the screen freezes.
Do you think it's a bad psu,  video card etc???
If it is an insufficient power supply, can recommend a power supply that would be enough and fit his case...Thanks.

Maybe any of following but your information is not enough to conclude.
 
1.  PSU problem; or  
2.  Due to overheat problem; or  
3.  Your 8600GT overclocked to unstable level, bare in mind that the degree of overclock ability is depends on indvidiual graphic card which is not 100% guarantee.
In my understanding HP computer set is normally with poorer cooling effect design.  
 
One of my computer set for my wife for BT and music editing with 8600GTS overclocked by Rivetunner 2.07 of core speed 740MHZ and memory Speed 1120MHZ is quite stable.

I had almost the same problem and it was power supply.
Actually the Atx12v 4-pin square cable that goes near processor to motherboard was faulty. It is necessary for additional power.

It's a factory overclocked card.  RAM is same speed...not sure about voltage timings.  GPU has a Zalman fan w/ max temp 53C via nv monitor.  He did remove all the old drivers b4 installing the new drivers.  

hey, i've got a 8600GT stock. in my case i had a no name brand 400Watt psu. in silkroad i always got artifacts when the grafics were more demanding. the solution was: i bought a 600WATT and the problem was solved.
 

Well, he's getting freezes and comp restarts...everyone in agreement the psu the  would be the best thing to replace?

I put my money on power supply... a decent QUALITY 450-500w unit  shoudl take care of your issue

vx550 gets my vote

Help!!!.....put in antec 500w earth??? and still have freezes.  pulled out adata 1gb x 2  ddr 400 ram, left the hp ddr400 512mbx2 ram sticks and evrything worked fine!!  But then, like a dummy i pulled out the hp ddr 400 ram, placed the adatas in those slots and now having problems booting up xp.  CAn't boot in safe mode or last known good configuration either.  Took out the adata ram and put back the hp ram and am having the same " no xp boot" problem!!! even w/ the original ram that had worked fine!!! please help!!!

try clearing the cmos and using the hp RAM. a-data has been known to have some bad chips. so that was probably the original problem too.