the answer to both of your questions is "limited by available system
meaning how big is your hard drive, memory ect. in your case with 512mb
memory, i would consider doubling it.
but i do like JLGWhiz's suggestion. assess would allow for better
managablity potential. you may have to do some coding, forming and querying
but the potential for better data management is greater than in excel. when
your data get that big, you may not have exceeded excel's limits but you may
exceed the human limit to manage with excel.