separate amp or upgrade HT receiver?

If you wanted a good one box solution buy an NAD T753.
It has all the power and features you need with the Q5's.
Biggest bang for the buck in the line.

It would be a better long term plan, however, to get something like a Parasound 5125 multi channel power amp. It would improve the sound now (not as much as getting the T753) but afford you much better sound later on.

Surround sound is not that demanding so you could eventually get a good 2ch pre-amp with HT pass-through. Whenever you feel the need for "better" surround sound you can just get a latest-and-greatest Denon AVR.

Surround sound receivers, pre-pros, and DVD players are outdated by the time they hit the shelves.

Good luck!

The single biggest improvement in my HT system using a receiver was to add an outboard 5-channel amp (in my case, a Sherbourn 5 -1500). I used my receiver as a preamp processor and was very happy with the sound. I would suggest at least considering that route.

Michael