You need to limit processes. If you are using a lot of the cpu(s) for one application, other unnecessary ones need to be disabled. Increase the amount of RAM. How much does the motherboard allow? Use that for the amount you want to have.
No, add another hard drive to the cable the size you want. If you want 300G then add a 300G drive to the cable and windows will accept and add it to the configuration, Keep the setting pin at CS(cable select)

Find a graphics card that is compatible with your board and be sure that you have a graphics library that helps. In the BIOS set the amount of RAM need for the video at ) and use the memory on the card. Go for 512M or more if you can. If only 256 is available then set the RAM in the BIOS for video at 256M. You could do 512M to add to the memory on the graphics card.