Trust me, you won't have any bad experiences with any of Intel's current line up. I didn't get any lag while doing heavy multi-tasking (Web browsing, gaming, mild Video-Editing, and movie watching) with an AMD Phenom II X4. That said, it is HIGHLY unlikely you would encounter lag with an i5.
On a side note, since you're using a Single Core CPU, any modern CPU would give you a HUGE performance boost. Going from a Single Core to a Dual Core will make you jump up and down in joy, going from a Single Core to a Quad Core, well, let's say you won't have any complaints for a long while :lol: Throw in an SSD into the equation and you would feel like you have your own personal super-computer!