As for the girlfriend situation, hey - what do I know? Just try and be careful with people's feelings, try not to place unreasonable demands on yourself or others, and have fun.
Telling the folks you are moving out...
Well, I don't know if my situation applies too well. I did the college thing two years after graduation I moved back home for about 6 weeks trying to get my stuff together. Both times I moved out, there was plenty of lead-up for my mom to get adjusted.
I'd recommend telling them ASAP and see if you can frame it about the awesomeness of starting a life with your GF-maybe-someday-wife instead of "god, we just need a place I can get naked and walk around in!"
Also - be realistic about your finances. If you aren't working full time, it is going to be rough. I don't know what you do for work or what the cost of living is in the South (cheap, I imagine) but again and again, money becomes an issue. It isn't easy (for most of us) being young and out of the nest in a depression / recession.