After looking at your CSS, I don't see anything that you can do. You might see if your client has ClearType enabled, if he's a Win/IE user, but other than that, it's a browser rendering issue. With Arial as your go-to font, it's hard to get more universal.
Clients usually aren't really amenable to the fact that sites and designs don't display identically across browsers and platforms, but you're in the unenviable position of having to make that argument. :-/