You'll be fine with the lower end Pentium series, but if there's a good deal on a Core i3 based system, by all means go for that. Anything over the Core i3 is likely to be wasted on you however.
I have a 7 year old Dell Dimension computer and want to buy a new one. I use QuickBooks, Quicken, and Microsoft Office. I also surf the internet but I have the slowest speed that Comcast offers. Thank you for any suggestions anyone can give me.