Single Crankset upgrade

My little Raleigh comfort has a 7-speed, 14-34T Cassette & single 40T crankset. We took her to the LBS to see if we could make it a double or triple, but the frame cannot be set for more than the single. However, I want to be able to have more power than my current 7 gears. Before clipless, I was averaging gears 4-6 and now 7 isn't high enough :eek:; I feel like I could push myself more if given the chance. I was looking at a 40-8T Cassette initially, but DH doesn't see me as getting what power/speed I want. I then looked at larger single cranksets; the largest crankset I have found is 48T. Would more teeth in a single crankset really help with that or is it a basic waste of money? On a crankset, what noticeable difference would you feel/see in adding 2 teeth, 4, 6...? Inspiration please!

How old is your bike? Does it fit well? Rather than go to the expense of upgrading it you may want to think about a new bike.

BTW, I run a 48T-34T compact on my road bike. I initially got the 48T because of knee problems but so far have found no reason to change.

You may be able to simply replace the chainring (not the whole crank). That would be your best bet. A 48T chainring will make all your gears 20% harder. If you never use the lowest gears, you could also consider a narrower-range cassette, so you have more gears in the higher range that you use alot.