I have built computers for many years. If you're on a budget you cannot beat AMD. I have had great success with them. Combined APU (CPU + GPU), pick a decent one, compatible MB, HD, 8GB RAM, you're all set.
You can use the case from your old PC but you'll need to get a new power supply (400W should be enough for you from Antec). If you're not adventitious. Then buy a case with a good power supply.
If you go over 4GB of RAM, you'll need a 64-bit OS. You can save more by not running windows (if you already have a copy you're ok). Linux as Steam supports games on it now.
Last edited by z00; 12-30-2012 at 05:36 PM.