Gateway DX4200-09 - Power Supply Troubles

I thought about bumping an earlier topic for this, but the last post was in January so I thought better.I'm currently looking for a compatible power supply for a Gateway DX4200-09. I've ordered two. The first was when I didn't do the research like an idiot and it didn't fit the form factor of my PC then it just fell apart after I took it to return it. The second was matched for form factor, plugs, the whole thing and I ran it by a hardware nut and was assured that it'd work. However, I couldn't get it seated in the case for some reason(it couldn't be seated securely against the back of the case).My question is, what sort of power supply can I put in? I'm looking for 500-550 Watts based on the new video card that I want to put in.The first power supply: http://www.amazon.com/dp/B0009Q56N6/The second: http://www.amazon.com/dp/B001L0N1M2/For those who are curious, this is the stock power supply: http://barebonekit.net/itemdesc.asp?ic=CSPGATDX4200090

if its a custom form factor then you cant change it. Manufacturers do that deliberately so you have to pay them to fix it or buy a new machine.

It's the same PC as this guy's: http://forum.romulation.net/index.php?topic=13495.0 And reading through, I couldn't figure out whether or not he got the power supply to work or not. >_>