Handbuilt wheels by The Master (theres one in your neighborhood, you just need to find him/her) are awesome. I ride custom sweet handbuilt wheels and they are the single most fabulous upgrade around. Besides the other eight or so most fab upgrades, that is. Handbuilt means handbuilt - if the shop brags about its wheelbuilding machine don't bother with them.
Alu bikes can be fine, and only you can judge whether it's time to move on. One possibility is new wheels and a carbon fork, which the boys in my shop swear by. If you want to move to a steel or carbon frame later, the nifty upgrades go with you.
Wheels are the ride. Even the finest factory set up bike in the world will improve with a great set of hanbuilt wheels. Mine were worth every cent.
About that carbon fork- MT bikes were originally alu with suspension forks to compensate for the stiffness. Not harsh, stiff. Good quality in a cross country ride. Alu for the road always seemed weird to me, steel is the stuff for pavement, but we've all noticed how fashion driven cycling is? Carbon fiber forks do for the road what suspension does in a hard tail, or so I'm told. Don't have one yet, riding steel with custom wheels that blow my mind. Gonna add the fork when my mileage improves.
So, the middle ground is wheels and fork.
Hows your back Goddess?

missliz