If you system is going to become;
ASUS P5G43-V WS with Intel Core 2 Quad Q6600 powered with ATX-500W-24P
According to the user manual located at
https://www.manualslib.com/manual/444938/Asus-P5g43-V-Ws.html?page=10#manual you will notice that your video PCIE Slot is actually 1 x PCI Express 2.0 x16, so this means any 2.0 or 3.0 video card will work with your system. GDDR 3 or GDDr5 doesn't matter
Now that said I looked deeper into your BIOS, and noticed that it was NOT UEFI (see page https://www.manualslib.com/manual/444938/Asus-P5g43-V-Ws.html?page=58#manual) this shows the old style BIOS and not the new style UEFI. which means I would stay away from any GTX 10xx series video cards as they require it.
finally your power supply, the CEMO atx-500w-24p is a very low end power supply, from what I gather from some research. it is made in China, and sold under a dozen brand names and all reported back this power supply can barely produce 240w on the 12v line. and in most cases the power supply does not have a PCIE 6+2 connector to use to power a video card.
Opinion. get a better power supply, 500w is plenty for your system, something like a 80+ Bronze Thermaltake or Corsair, this factor is important, as the power supply that you currently have limits severely the choices in video cards you can use. for example;instead of a Gtx 970 for example, you are limited to a GT 1030 or RX 460, which in the end are my recommendations to your current system if you do not want to change your power supply.
Best video card I can see you running; GTX 750TI but that means a new power supply for your system as well, alternate choice to the GTX750TI , you could find a Used HD78xx Video card (HD7850 or HD7870) but only if your PSu has 1 PCIE 6Pin connector .
why? because a duo core2 Q6600 on a DDR2 Motherboard will struggle at best to run a game like call of duty, never mind running any AAA games out there, can you run a 1050ti on your system , probably, maybe not because of your bios, but if it did your CPU would be a huge throttle of data, and you would not get full performance of that video card, pairing it with a video card that is better balance for your cpu is a better idea in the end.