Just for your reference a 17" HP laptop using A6 will be about 1.5 to 2.5 times the performance of your machine and it cost under $500. I just bought a sub-$400 Celeron version for remote CCTV and viewing of my business data online.
A SSD will drastically enhance your bootup and firing up of software. A small one will suffice if you are not adding too many program. Newegg has a sale yesterday or so, 120G under $60 but I think it's gone now. 60G is about $40. Keep your old HD as storage and use the SSD to boot and run your Photoshop on it. You will see night and day difference when swapping between larger pics. Make sure you check the connectors of your old PC whether it has a SATA connector for the SSD.
Nowadays, instability are mostly due to software issue such as missing some Direct X, some Java, and so forth for the PC world. If you are not a power user stay with pre-build ones that comes with all the legitimate software licenses to update. The pre-build ones are not expensive-just a bit limited in configuration and future upgrade ability.
The first PC I ever build is CPM 80/Apple compatible;
The first PC that has a hard drive I managed is the XT and YES everyone in the computing world at the time love it; How dare IBM sold the HD division along with its patents out!!!
The first networking system I build was with TRS 80 for a school.
@Chief
TRS 80 does have FPV tactical war game, like the "Tank", primitive strategic war game "Battleship"(non-commercial). LOL