Maybe I am just cheap, but here is my thinking. Why upgrade 1 or 2 cogs in the back? For about the same amount of money you can put another ring on front and get 5, 6, 7, 8, 9, 10, heck even 11 more gears depending on what's out back! That's a lot better deal in my book. Then you can get a really close ratio cassette and not be stuck with those wide gaps either.

Case in point: I bought my wife a 1992 Team Miyata for Christmas. It had 7 speed 105 with a 130 hub. I bought the following on Ebay:

NIB 8 speed hub body $15.00 NIB FC-1057 105 road triple $65.00 NIB triple FD $20.00 NIB 12-23 and 13-26 HG50 cassettes $23.00ea. I could have used the same chain but put a new one on for another $15.00. It took about an hour to install the parts and set the new FD. 8 more speeds and the other cassette for around $150.00 is hard to beat!
I forgot about the longer BB spindle. Add another $10.00 for that.

I also replaced the downtube shifters with the new Claris STI shifters. $40-45.00ea if you buy them separately online. Brand new 24 speed 105 drivetrain and STIs for that amount of money is a sweet upgrade indeed.