I guess I am in the minority here. I think the answer depends on where you want your system to be in a year or two, and how much you care about music vs. HT.
I added a Bryston 4BST to a mid-fi Onkyo receiver with the plan to add a separate preamp when funds permitted. My experience was that there was a noticeable, but relatively small, improvement (sound had a greater sense of ease, slightly more bass). However, it wasn't until I added a separate preamp that I noticed a big improvement. This is all in reference to music as I use my system for HT only rarely. If you take a long-term view and plan on continuing to upgrade your system, I think adding a power amp as you are thinking, then adding the preamp when you can, is a great idea. Just don't salivate over the expected improvement. Good luck.
I added a Bryston 4BST to a mid-fi Onkyo receiver with the plan to add a separate preamp when funds permitted. My experience was that there was a noticeable, but relatively small, improvement (sound had a greater sense of ease, slightly more bass). However, it wasn't until I added a separate preamp that I noticed a big improvement. This is all in reference to music as I use my system for HT only rarely. If you take a long-term view and plan on continuing to upgrade your system, I think adding a power amp as you are thinking, then adding the preamp when you can, is a great idea. Just don't salivate over the expected improvement. Good luck.