You should be able to multibox 2 accounts with the 32 bit client - which only uses 4Gb RAM. You may want to back off your graphic settings to medium to see if that helps. With 8Gb RAM or more you should be able to use the 64 bit client.
If you don’t have 2 monitors, a multi-client switcher like EVE-O Preview v5.0.0 "Scourge" (multi-client preview / switcher) [ 2019-07-12 ] may help as well.
If you plan to keep your current system for a while an SSD is a worthwhile replacement for a mechanical hard drive. If you’re thinking of replacing the machine wait and make sure the new one has M.2 NVME storage.