IB isn't a must have. SB is a tad slower and cheaper. If the 2500K is a lot cheaper compared to the 3570K, get the 2500K. But, in your case, best to get a new system. HP and Dell use non-standard mobo and case sizes, so then you'd have to get a new case, and at that point, you're not that far from getting a totally new computer.
Yeah, if I went I5, Mobo, and new case, It's around 500$. Don't really want to spend that much. I worry about buying new systems though. Not sure it could be as good, are there any around 500 you would recommend?! I would also like to upgrade in the future, so I guess I would need to align myself with a system that would be good for intel upgrades... I could return my newly acquired upgrade pieces!
I still think there is something wrong. No system like that should have issues playing DVDs. I played my first DVD on a Pentium 1 @ 300MHz and a 4MB PCI video card. If things stutter then its something else.