I happily used a tarp and bivy combo for years - until I saw the first solo tent that was all mesh with a rain fly. It weighed the same as my tarp and bivy, and I never looked back. The ability to sit up, change clothes, and do chores inside the mesh is priceless.

I agree with Franco and Mouse: get a light solo tent. My own choice is the MSR Hubba NX; they also have a lighter variant called the Carbon Reflex 1 that might work. If you want a bit more room, also check out the TarpTent Rainbow. (It's a single wall tent, which doesn't work for me in the hot, humid Ohio River valley, but that's a personal thing. It's well made, well-designed and, as I recall, weighs about two pounds all in.