Hi,
According to the specifications in the [https://downloads.dell.com/manuals/all-products/esuprt_desktop/esuprt_dimension_desktops/dimension-5150x_service%20manual_en-us.pdf|service manual] the motherboard only supports 4GB of DDR2-533 RAM.
There are posts online that show that 8GB will work in a Dell E510.
Also to consider is whether to use Win10-32 bit or 64-bit. Win 10 32-bit won’t recognize more than 4GB of RAM so installing 8GB is a waste. Win 10 64-bit will recognize up to 128GB ''but'' you would have to check that the CPU in the computer supports 64-bit, some Pentium 4 CPU models don’t
I run Win10 32-bit with only 2GB of RAM (non upgradeable) on an old laptop (pre 2010, Intel Atom CPU) . It is painfully slow but still works OK and can do most things albeit slowly. Installing a SSD instead of a HDD, will no doubt improve the performance a bit, but I cannot justify the cost.
If you do decide to upgrade/update then ensure that the motherboard [https://www.dell.com/support/home/en-hr/product-support/product/dimension-5150x/drivers|BIOS] is the latest version, just to make sure that it may be compatible as well.
When Win10 is installing it should provide the most suitable drivers for the hardware as the Dell drivers on the computer’s support page are outdated as far as Win10 goes. Support from Dell ended with Win Vista.
In the end it may come down to economics. It is a very old computer in computer terms and it may be very slow and may not even work when trying to use 3rd party programs designed to run on computers with more recent and better specifications.