Ensure you reference your service manual before carrying out upgrades:
The most recommended upgrade for this system is a Solid State Drive, here is my affiliate link to Crucial here. If purchasing a SSD or Memory Upgrade please use these links as I will get a tiny bit of commission which will help fund my guides.
SSD Drive Upgrade
Use the Crucial system scanner on this model as there are two variants of motherboard and one can support more RAM than the other.
Please ask on the Dell Community Forums for the latest recommendation for Graphics Cards. Ensure to include your form factor when asking for a Graphics Card Upgrade Recommendation.
It is recommended to install Windows 10 on this system see Windows OEM FAQs and Downloads. The Windows Vista OEM or Windows XP Product Key won't be compatible with Windows 10 Installation Media however you can run Windows 10 Unlicensed. Windows 10 will have most of the system drivers required inbuilt however you can install the Windows 7 Drivers if you have any unknown Devices in the Device Manager.
Drivers and Downloads for Windows 7
The two different motherboard variants each have their own BIOS Update. Only the correct BIOS update will install on your system, the other update will warn you, your system is incompatible.
System Utilities (Windows 7 Only)
Chipset (Windows 7)
Video (Windows 7 Only)
There are three variants for video, the onboard Intel graphics or dedicated AMD or nVidia cards.
Communications (Windows 7 Only)
Not all systems have a modem. If the Driver fails to install, there is no need to try and install the rest in this category.