Which Xbox 360 should I get?! 4gb or 250gb?

So my old Xbox 360 got the rrod. Iv fixed it soooooo many times, but it got it again and I'm fed up with trying to fix it. Should I get the 4gb Xbox 360 slim Or the 250gb Xbox 360 slim for another $50 By the way, I have a 20gb harddrive. I thought that if I took it apart and took the hd part out and put in the 4gb slim it would be just fine.