The 6200 is a pretty weak gamer. Going to a socket 775 for PCI-E will entail a few changes - and there aren't too many 775's that will accept standard DDR ram. Why don't you just go with what you have there and upgrade the video card later if needed? You will be able to get decent AGP cards for quite a while longer. If you want one now - look at this one:
You can get a case without a power supply pretty cheap - one that you like the looks of - and buy a decent separate power supply. However, there is not a cheap case out there that has the construction quality of an Enlight - it's built like a tank.