A 32-bit system can recognize up to 4GB of "memory." Most people take this to mean the System RAM, however it means all devices which must be "memory mapped."
- System RAM
- Graphics Cards
- other devices.
If you upgrade a video card from 512MB to 1024MB then you will lose 512MB of available RAM from your installed 4GB of RAM.
So with a 512MB video card you might have between 2.8GB and 3.3GB of your 4GB installed memory available.
I wouldn't worry about things too much though. The amount of RAM left over after a 512MB or 1024MB video card installed is still quite adequate.
In the future, your general Windows experience would be improved by having Windows on a Solid State Drive. It's worth waiting, but in a year or so consider CLONING your hard drive to an SSD if you think it's worthwhile (opens programs and starts Windows quicker.)
1) You should always HIBERNATE or use SLEEP to turn off your PC. Don't turn OFF (too slow) and don't Standby (loses data if power goes out).
2) You should make an IMAGE of your Windows drive occasionally in case your hard drive fails, or is corrupted. Acronis True Image is a great program and there's a free version I like from Western Digital. You must have a Western Digital drive for this to work (even a USB drive from WD).