is dependent on RAM capacity and also on how powerful your video card is. Another speed limitation is the source hard drive, whether it is a HDD (traditional), SSD, or hybrid drive. The former is the slowest, and the first latter is the fastest. Hybrid is in between for data throughput speed. It is dependent to some extent to CPU speed, but the hard drive is your main bottleneck in starting/running programs as the data throughput is much slower than any RAM will ever be.
Changing your video card to a more powerful model will likely mean the PSU will need to be upgraded to higher wattage output than the PSU now in the system to be able to support the new card.
For your needs, replacing both DIMMS with 4 GB DIMMS x 2 is the cheapest way to go, short of replacing your system with better memory specs, hard drive support, and a more powerful PSU to run that video card. Each card is a 4 GB card, to be clear about that, no misunderstandings.
Be cheap and live with what you have, but replace the existing DIMMS with the proper ones.
Remember, once upon a time, 32 MB RAM was enough.