djengala

Okay, what are the specs of both PCs? Usually you cannot just move a OS to a new PC without clean installing it.
 
The way it's sounding though, it sounds like PC 2 is in UEFI mode and the Windows 7 drive is configured to run in BIOS (Just my guess).
 
I would heavily recommend cleaning installing Windows 7 onto PC 2.

you need an active dp adapter and another dvi to vga adapter 

I tried to do the same thing as you did and ran into the same issue. Turns out that for 3-monitor Eyefinity, you NEED to connect one of the monitors using DisplayPort. If your monitor doesn't have a DisplayPort input you will need to use an ACTIVE DisplayPort adapter.

Yes. The other two can be used with HDMI, DVI or VGA doesn't matter, but at least one has to be on DisplayPort.
 
However again big emphasis that if your monitor does not support DisplayPort you will NEED an ACTIVE DisplayPort adapter. A passive one will not work for Eyefinity. 
 
I wouldn't do Eyefinity in the first place though. It looks horrible with different size and resolution monitors.

It's not just about wattage, it's about PSU quality as well. The ones I mentioned are of good quality as well. Finding a high-quality 350-450W PSU without paying the same price for a 500-550W PSU is near impossible

PSU's to look at (Varying prices/modularity, check the connectors):
 
XFX XTR 550
XFX TS series (550)
EVGA 500B
Seasonic S12II
 
@djengala

with 550W he should have plenty.

The Q9300 is the best. I wouldn't pay more than 50 bucks for one though.

Q 9300.
 
What is the price in question?

No. It is not recommended. Not only your current PSU is based on a rather archaic design, using a secondary PSU for a +12v power draw can be problematic for certain PSU. Many low-end PSUs require a load between the 12v and the 5/3.3v to be within a certain ratio for it to operate properly. If it is not properly "balance", then you will see a severe voltage drop on the 12v rail and spike on the other rails. This is similar to the whole "Haswell capability" with certain PSUs.
 
You will need to invest in an independent / DC-DC regulated PSU if you want to do this, but for your system with such a modest power draw, you can just avoid the hassle and power it with that one PSU.
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The "500w" minimum requirement is for the entire system, which Sapphire nor nVidia didn't clarified what sort of components are included in that estimation. Is it a system with an i3? Or a system with a LGA2011 i7? Also, does that factor in a 500w PSU that are rated for continuous or peak?
 
https://forum-en.msi.com/faq/article/printer/power-requirements-for-graphics-cards
A retired forum staff at MSI stated that that the 7870 requires a minimum of 500w as well; however, it also states that it needs to have at least 23A on the 12v rail as well. That's only 276w. Both the Antec Earthwatt Green 380w and Seasonic G 360w has met that amperage rating and higher, in which shows that these manufacturer is also giving some leeway in their recommendation to compensate for the poor, overrated units.
 
http://www.techpowerup.com/reviews/Sapphire/HD_7870_Flex/26.html
In accordance to that link, W1zzard has used a $2000 equipment to isolate the power draw of the Sapphire HD7870 Flex GHz Edition to a max power draw of 150wDC during Furmark. Unless you are running some odd build with two OCed CPUs or something, a good 400w PSU would be plenty (yours is not a good 400w unit). I would suggest getting a new single PSU like the Corsair CX430v3 if you need something inexpensive. But to give you any sort of recommendation, it would be helpful to know your budget and links to stores you are able to buy from.

Australia has expensive components as well.
All I was trying to say was, save up and get a better PSU, because buying a cheap PSU could later down the road, destroy your components.