Dell Dimension E520 Upgrade

You would be better off abandoning trying to update the old Dell and put the money into a new PC. Even if you upgrade it (and it will be limited) you will still have the old technology E520. This is an obsolete BTX style motherboard (I had an E510 with a D925CPU). Replacing the motherboard is just about out of the question as there are very few BTX style motherboards available and Dell uses a proprietary and undocumented front panel connector, not the standard "ATX" connections so that too limits using a different motherboard.

With only 4GB max, you won't need the 64 bit OS as the big feature of a 64 bit OS is that it can address more than 4GB of RAM. If you use a 64 bit OS, it will address the full 4GB but you will still wind up with only 3 to 3.5GB of useable RAM, the same as you will get with a 32bit OS.

You can upgrade the Video card, but you will have to use low power video cards unless you replace the Dell power supply with a higher wattage model. The Dell power supply does not have a PCIe power connector that is also needed for many newer video cards.

But the best "bang for the shilling" would be to upgrade to the full 4GB of RAM, add a new video card and power supply. The E520 uses a standard ATX style power supply, its not a Dell proprietary. A "modular" type power supply is recommended as it eliminates most unused power leads (you won't have to stuff them inside the case). Crucial Memory is recommended as it is guaranteed to work in Dell's. Dell's have a history of compatibility with non-Dell memory and the reason for recommending Crucial. I used to do a lot of support on the Dell user forums and all the techie's there will recommend Crucial for non-Dell brand memory. Other brands can be used but its hit or miss. Avoid the Kingston Value Ram (KVR).