I have the money to buy a new HD500X, but since a used HD500 is available for cheaper in my area ($300 dollars)...but i was wondering if there's significant differences between the two units besides the improved footswitches and DSP?

I mean, if i buy the the used HD500 i would be saving money but i don't know if i would be missing important updates or even new amp modelers that might be available for the HD500X?
No way to know regarding updates, but the DSP allows you more freedom with the number of FX you can use at once, etc. I think you're looking at an incremental dose of DSP power, and I don't think there are going to be amp models that you'll miss out on by having the HD500 over the HD500X. If it's me, I'd rather save the bucks, sit out the 500X and then move on the next version after that.